


Taxonomy

by Lindentreeisle (Captainblue)



Category: Harry Potter - J. K. Rowling, Sherlock (TV)
Genre: Alternate Universe - Fusion, Case Fic, Gen, HP characters do not appear, Magic
Language: English
Status: In-Progress
Published: 2012-05-16
Updated: 2012-05-25
Packaged: 2017-11-05 11:21:39
Rating: Teen And Up Audiences
Warnings: No Archive Warnings Apply
Chapters: 2
Words: 37,786
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/405836
Author URL: https://archiveofourown.org/users/Captainblue/pseuds/Lindentreeisle
Summary: <blockquote class="userstuff">
              <p>"Amazing," John says. He's not sure why he's so impressed by someone ferreting out one of his most embarrassing traits so easily; perhaps it's because no one has ever done it that quickly without any input from John. Or perhaps it's because Sherlock has yet to give him that look of pity that witches and wizards- even his parents, even Mike- always give him when they talk about squibs.</p><p>Sherlock stares at him. "What?"</p>
            </blockquote>





	1. Chapter 1

**Author's Note:**

> Thanks to Lawless, theSardine, and Lady Ganesh for betaing.
> 
> This fic would not exist without Sardine, who read my first attempt at a HP/Sherlock fusion and told me to write it again properly.
> 
>  **On canons:** You do not have to be a Harry Potter expert to follow the narrative, but some basic knowledge of the universe will be helpful. For the purposes of this AU's timeline, the events in the HP books occurred exactly as JKR wrote them. The only difference is that in this universe, everything has been time-shifted exactly 11 years forward. So, for example, Harry Potter was born on July 31, 1991. The Battle of Hogwarts occurred on May 2, 2009. Finally, a special shout-out goes to [the Harry Potter wiki](http://harrypotter.wikia.com/wiki/Main_Page), which was an invaluable fact-checking tool.

**squib** (skwib) _noun_  
1\. a non-magical person born to at least one magical parent  
2\. _Muggle usage_ a. a small firework that burns with a hissing sound before exploding; b. a short piece of satirical writing  
3\. _Informal derogatory_ a valueless or unwanted person  


    

— _Oxford English Dictionary Wizarding Edition_

    

When Mike leads John Watson into the teaching lab at St. Bart's to see a man about a flatshare, a tall, dark-haired stranger looks up from his microscope and gives John a rapid once over before opening his mouth and saying, "A squib." His tone is matter of fact, as if he were announcing the weather, but John rocks back on his heels and contemplates the many, many ways in which he could end Mike Stamford's life.

It's a fleeting thought; John darts an accusing glance at his friend, who just grins, and John decides that Mike wouldn't do that to him. He's known Mike since they were boys, their dads went to Hogwarts together, they were best friends at Bart's. Mike can be shockingly obtuse- he'd gone as infuriatingly pitying as everyone else when he saw John's cane- but he's also as guileless as a puppy. He would not go sharing intimate and humiliating information about John with perfect strangers.

So how the hell _does_ the man know that John is a squib? He's chilled by the familiar, irrational fear that somehow it is writ large on his face: a source of shame that he always tries to keep under wraps as long as he can manage.

"Interesting thought, Mike," the stranger says, his tone now musing. His eyes narrow. "Ye-es. I believe that would suit."

"What do you mean, suit?" John says cautiously.

The man's eyes snap back to him. "Flatmates," he says briskly. "You need one, I need one. Clearly that is why Mike brought you here, as I only mentioned it to him this morning. I've found a nice place in Central London, convenient for my work. Yours too, I suppose, if you end up taking a position at Bart's as Mike has offered to find you."

Mike _did_ offer to find John a position, on the cab ride to the hospital, despite John's rubbish leg and his intermittent hand tremor. He "knows a guy" in staffing, just as he had known a guy who was looking for a flatmate. Mike knows everybody and likes everybody; it's how he gets away with being so wildly unconventional as to live and work in Muggle London instead of raising his family properly in a wizarding enclave. Mike's wide circle of half-outcast wizard buddies is the only reason John had any close friends at all when he was at Bart's. But again, how the hell does this guy know?

"I'm very busy just now," the man continues. "We can meet there tomorrow at about four. Let me give you my number; do you have your phone with you?"

John feels almost dazed as he fumbles out his mobile. The phone is new to him and has too many menus.

The stranger huffs impatiently. "Here, let me," he says, and deftly plucks it out of John's hands. John barely has time to become annoyed at the cheek of it before the man's thumbs have finished dancing over the keyboard and he's handing the phone back. John flips the screen right-side-up and sees that the man has entered himself into John's contacts under the name _Sherlock Holmes_. Weird name. Weird man, though, so maybe it suits him.

John looks up to see Holmes studying him with a thoughtful expression. "You're a doctor," he says.

"Yes," John agrees cautiously.

"Fifty-five year old female, found dead in her home. She has no history of major or chronic illness aside from a childhood bout of appendicitis. The cause of death is sudden, massive organ failure. Her organs were, according to one observer, quote liquified endquote," Holmes rattles off, almost with relish. "Method of death?"

John rocks back on his heels. Half of him wants to snap that he is not a performing dog, nor is he a medical student. But the other half of him is stirring with interest. "External injuries?" he asks.

Holmes' mouth quirks a bit, as if the dog has just done a droll trick. "Minimal. Bruising, but no lacerations and no projectile wounds."

"How much bruising?" John asks. "Because taken off-hand, I'd say blunt force trauma to the abdomen. Would take a hell of a lot of force to do that much damage."

Holmes spins back to his microscope and rifles through a pile of paper sitting next to it, then slides a handful of sheets across the lab table toward John. "Do these support your theory, doctor? Holmes asks." John steps forward to examine them: they are color copies of photographs, two per page, showing the torso of a young woman with extensive, ugly bruising that covers her entire abdomen. Except-

"This isn't proper bruising," John says, sliding one sheet aside to look at the next. "It's blood pooling under the skin, obviously, but it's too diffuse. It can't have been caused by punches or blows with an object. Unless it was one enormous object and it hit with enough force to do all the damage in one go. Was she found in a factory or something?"

"In her sitting room," Holmes says.

"I'm afraid I'm stumped then," John says, setting the sheets he's holding back on the table. "Sorry, pathology isn't really my field."

Holmes frowns. "But you're an army doctor, surely you've seen plenty of traumatic injuries," he says.

John frowns right back. "It doesn't take an expert to diagnose a gunshot wound," he snaps, then does a double take. "And how the hell do you know I was an army doctor?"

"The same way that I know you're a squib from a family of wizards that doesn't approve of your career, that you were recently discharged after being wounded in combat, and that you have a therapist that you ought to fire outright." Holmes picks up a ridiculously posh coat and scarf from a nearby stool and quickly slips them on, gathering up his papers as he does so. "I'm going to be late. I'll see you tomorrow, yes?" Holmes raises a hand in acknowledgment to Mike, and sweeps out without waiting for John's answer. 

John stares at the door for a second, bemused, before he realizes that not only did he never get a chance to give Holmes his own name, he has no idea where this flat is that he's supposed to visit. As if in response to the thought, his mobile chimes with an incoming text.

 **Message received**  
221b Baker Street. SH

* * *

The rest of the day is the usual barrage of trivia his life has become. To the shop for Pot Noodle and tea bags. To the post office to mail a letter to his mum, who has become even more traditionalist in the past year and now refuses to use telephones. To the Pension Committee office for another thrilling round of paperwork. It's all incredibly boring, yet John manages to find excuses to stay out until his leg is aching and he's hungry and exhausted and he has to go back to his grotty little MOD-sponsored flat. 

After eating, he spends about ten minutes staring at the blog his therapist insisted that he start. A blog, that's a joke. As if he can write down half of what he's thinking or feeling, or even half of what happens to him, when it's so tied up in the wizarding world. He can't even talk to the therapist about it. John pecks out "This is a complete waste of my time, and my time is almost worthless," then deletes it and doesn't post anything at all. 

To his surprise- most wizards aren't internet savvy at all in his experience- when he googles Sherlock Holmes the top result is a webpage titled "The Science of Deduction," apparently written by the man himself. The page claims Holmes is "the world's only consulting detective." Believable, since this is the first time John's heard the phrase. He goes back to the search results and checks out a few of the other links. A couple of newspaper articles about criminal investigations briefly mention Holmes, though they don't make clear what his role in the cases was. There are more than a hundred results from various forums and MySpace pages; John visits a dozen or so and finds Holmes mentioned a number of times, usually alongside the words "tosser" and "fucking arsehole." 

John has most of the next day to work himself up to the idea of going out and meeting Sherlock Holmes at 221b Baker Street, and he still almost doesn't do it. Even from what little John's seen, he can tell that the man really _is_ kind of a tosser, dismissive and arrogant. John isn't sure he wants to live with a wizard again anyway; it's easier to live with Muggles, even if he has to hide a lot about his family and his upbringing, because he knows they're not constantly pitying his sad, magic-less state. But Holmes did seem interesting, and John wants to know how he knew all that stuff about him. And John is a soldier for God's sake. He'll be damned if he's going to hide in this crummy flat like an invalid shut-in.

He leaves at three, to make sure he'll be on time, and makes it to the flat with fifteen minutes to spare. He waits outside for a few minutes and then, on impulse, he rings the bell. A kindly, crinkle-faced older woman opens the door for him. "Erm, hello," he says awkwardly. "I'm supposed to be meeting a bloke here at four, I just thought-"

"Oh, Sherlock said you'd be along!" she says brightly. "I'm Mrs. Hudson, the landlady. It's nice to meet you, dear. You come right with me." She ushers him inside a foyer with a frankly alarming wallpaper pattern and chivvies him up the stairs. "I hope this won't be too much for your poor leg," she says, and John grits his teeth to hold back a scathing reply.

When he reaches the top, he's standing at a junction with two doors. One in front opens into a sitting room, and to his left is a smallish kitchen that looks like it might be contemplating a second run in life as an apothecary, or perhaps a chemistry lab. There are two cauldrons going on the stove, a jumble of laboratory glassware piled in the sink, and a debris of books and random jars and heaps of dry potions ingredients spread across the small kitchen table. The only things missing are the quill pens and rolls of parchment that wizards typically use for taking notes.

"I'll leave you boys to it," Mrs. Hudson says delicately. "You just let me know what you think, dear, and we'll have another set of keys made if you like the look of the place."

"Thank you," John says, wondering what exactly _it_ is that they're being left to. 

The man in question glances up from one of the books he has been studying intently. "Ah, good, you're here," he says. 

"Erm. Hello," John tries. Holmes digs amongst a squadron of empty Skele-Gro bottles on the table and locates a box of nicotine patches, then rolls up his right sleeve and carefully adheres a patch alongside the one that already adorns his arm. John notes that he doesn't take the old one off. "Trying to quit smoking, then, Mr. Holmes?" he says.

"Never started," Holmes says. "The nicotine helps me think. Of the other legal stimulants, caffeine is also effective but at high dosage levels, tolerance adaptation is far too rapid. And call me Sherlock _._ "

John wonders at the emphasis placed on _legal stimulants_. "Okay. I'm John. John Watson." Sherlock makes a noncommittal noise. Perhaps it's time to change the subject. "This is a nice neighborhood," he says. "I would think that with a spot like this, the flat must be fairly pricey."

"No, Mrs. Hudson's giving me a special deal," Sherlock says. "She owes me a favor, as I helped dispose of her husband." John's face must reflect his alarm at this point because Sherlock hastens to assure him, "Oh, no, no. He was wanted for murder in Florida based on a series of rather nasty broomstick hexes. I was able to provide the necessary evidence."

"Oh," John says. Sherlock looks back at his book, apparently disinclined to give John any kind of tour, so he takes a few steps into the sitting room and has a look around. "This could be very nice," he says, because it's a lovely place even if there's crap everywhere, on every surface, in heaps on the floor. "It's, er-" a complete tip. "It's a bit- cluttered."

"I haven't finished unpacking yet," Sherlock says. "Had to move in rather a hurry, I'm afraid. There was an incident at my old flat."

Incident? John asks, raising an eyebrow.

"An experiment," Sherlock says dismissively. "Some overreacting fool called in the Oblivators in the aftermath. As if any Muggle was capable of deducing the true cause of the disturbance, even with his idiot brain fully intact."

John's spine stiffens and he pauses for a moment. "You know," he says as neutrally as he can manage. "My dad's Muggle-born." Most of his Muggle extended family is dead or estranged, but that's hardly the point. John has been living and working shoulder to shoulder with Muggles most of his life; hell, he's practically one himself.

Sherlock looks up at John and blinks rapidly, looking blank for just a moment. "Oh!" he says. "I see. No, you misunderstand. The vast majority of people are idiots; the ability or inability to do magic doesn't enter into it."

"Ah," John says. Just a general disdain for humanity, not bigotry. Why does that feel like such a relief? "And you don't count yourself among the idiot masses, I take it."

Sherlock's mouth quirks up at the corners, the barest ghost of a smile. "I'm a genius," he says matter-of-factly.

"Bedrooms?" John asks Sherlock, because okay, it's a _very_ nice flat, and he can put up with cynicism _and_ narcissism if he has to.

"Hmm? Oh, yes," Sherlock says. "Mine's back through the kitchen next to the bathroom. You'd be upstairs. Have a look, if you like."

John shrugs, sets down his cup, and does. It's just a room- bed and desk and wardrobe, with a window looking out the back. It's slightly bigger than the MOD room he's living in now, if even more barren. He finds himself mentally unpacking his few belongings and deciding where to put them, and gives himself a sharp mental slap. Is he really going to move in with a total stranger just like that? A man he knows nothing about?

John stumps back downstairs, to find that Sherlock has abandoned the kitchen and is sitting on the sofa in the sitting room, tapping on the keyboard of a trim little netbook. Still not a quill or a scrap of parchment in sight, much less a wand, which is startling given how much reliance your average wizard places on his wand. "I usually keep it in the inner pocket of my suit jacket, but it's not there just now," Sherlock says without looking up.

"What?"

Sherlock shuts his laptop and looks John in the eye. "My wand. You keep looking for it." John flushes and opens his mouth to protest, because he isn't even doing it consciously, but Sherlock goes on, "Quite right to do so; it's always best to know where the weapons are." He starts rooting in the sofa cushions. "Ah, here." He draws out a long, stiff-looking wand with some black scorch marks at the tip and rather a lot of smudges over the length of the light colored wood. " _Betula pendula_ , silver birch, fifteen inches, unicorn hair. Would you like to see?"

He makes as if to toss the wand over, and John's eyes widen in shock as he crams his hands into his pockets. "No, that's all right," he says hastily. Harry and his parents always went berserk if he messed with their wands, and every wizard or witch he's ever met has treated theirs like a personal treasure or a dangerous tool. Who the hell leaves his wand in the sofa cushions and then tries to toss it to a near-stranger like a piece of fruit? Sherlock shrugs and sets the wand on the coffee table. "You know-" John begins, and then stops. "Of course you know, you told me." John rubs the back of his ear nervously and meets Sherlock's calm gaze again. "How did you know?"

"That you're a squib?" John is ready this time and he thinks his instinctive flinch is invisible. "Solider and doctor are both unusual careers for a wizard, and I already knew none of Stamford's circle of wizard friends had need of a flatmate. Yet when I approached, you were scanning me as if for weapons, looking only at the sleeves and chest, the most likely hiding spots for a wand. Together it means you're living as a Muggle despite being raised in a wizarding household. Add in the lack of a wand callus on the forefinger of your dominant hand, and there we have it. Simple."

"Amazing," John says. He's not sure why he's so impressed by someone ferreting out one of his most embarrassing traits so easily; perhaps it's because no one has ever done it that quickly without any input from John. Or perhaps it's because Sherlock has yet to give him that look of pity that witches and wizards- even his parents, even Mike- always give him when they talk about squibs.

Sherlock stares at him. "What?"

"How did you know that I'm an Army doctor? And- the other things?"

Sherlock presses his palms together and rests the tips of his fingers under his chin. "Your obvious familiarity with the laboratory environment, along with your attitude toward Stamford, say trained at Bart's. Therefore a doctor. Your hair and posture are undeniably military, so Army doctor. You're tanned but not below the collar or above the wrists; you immediately check the room for exits and the people for weapons upon entering; you limp badly but you don't lean on your cane when you're standing still. Conclusion, recently in combat, resulting in traumatic injury, plus a limp that's at least partially psychosomatic."

"My family," John reminds him.

"Oh, yes. Well obviously they disapprove of your career; that's just the balance of probabilities." Sherlock smiles faintly. "And you should fire your therapist because you'll never be able to tell her any of this and any advice she provides will be based on insufficient data and therefore useless."

"Incredible," John says. "Just…brilliant. And you got all that from looking at me for, what- ten seconds?" 

Sherlock flushes and looks mildly pleased. "People don't usually like hearing my observations," he admits. "They tend to think I'm…odd."

"Yes, well, they think that about me as well," John says. He gets it from both sides- wizards because he can't do magic, and Muggles because he knows too many things that they don't- which seems especially unfair.

There's a battering at the front door, and Sherlock jumps up and goes to look out the window, tucking his wand into his jacket and buttoning it up. "Oh," he says with evident pleasure. "The police are here."

John has no chance to ask him why that would be good news, as there's a pounding of footsteps on the stairs and a gray-haired man clad in an overcoat appears in the doorway. "Well?" Sherlock says imperiously.

"There's a third one," the man says. Just like the others, no immediately obvious method of death and pretty much identical circumstances.

"I'd like your opinion on the scene," the man says. "There's probably not much you can tell from the body itself without being a doctor. Have to wait for pathology on that."

Sherlock's face lights up, and he looks directly at John. "What do you say, Dr. Watson? Want to come take a look at a crime scene?"

The stranger seems to notice John for the first time. "Hey, _no_ ," he protests. "This is not a bloody excursion. You don't get to ask friends along. I could probably get sacked just for bringing you."

"No you couldn't, your superiors know my track record as well as you do," Sherlock says. John would be prepared to describe his tone as "gleeful." "And he's not exactly a friend. This is Dr. John Watson, lately of the Royal Army Medical Corps. Doctor, Detective Inspector Gilles Lestrade."

John finds himself automatically stepping forward to shake the DI's hand. Lestrade glances at John's cane and then meets his eyes. "No offense, Dr. Watson," Lestrade says. "But whatever he's told you, Sherlock does not run New Scotland Yard."

"Clearly," Sherlock sniffs. The implication that he would be doing a much better job of it is as clear as day. "However, if you want my opinion, you must allow me the…tools…I need to make it an informed one."

"Absolutely not," Lestrade snaps.

"All right then," Sherlock says, sitting down on the sofa and reopening his laptop. "Of you go. Enjoy the inevitable press conference. I have plenty to occupy me here." He locks eyes with the DI, and John gets the sense of some enormous battle of wills occurring between them.

"Fine," Lestrade says. His glare is hot enough that John is surprised Sherlock doesn't spontaneously combust. "But I'm not a car service. Follow me in a cab. 86 Sudbourne Road." Lestrade pounds back down the stairs without awaiting an answer.

Sherlock waits until the front door closes before he leaps up and practically _runs_ to the hook for his coat, chuckling with delight. "Oh, brilliant!" he says. "The third linked death this week, _has_ to be murder, and this time they've actually called me in early enough to see the body _in situ_." He whips the coat on and pauses at the door to look at John, who hasn't moved. "Well? Are you coming?"

"Coming where?" John asks.

"Crime scene! Obviously." Sherlock is practically vibrating with impatience. "Come on, have a look at a corpse with me. Or you can stay and watch my mallowsweet reduce. Completely up to you."

Or he could go home, but John finds he really doesn't want to. The novelty is tempting- not so much looking at a corpse, but visiting a crime scene with a so-called "consulting detective" on invitation from the police. A chance to see what a consulting detective does. Maybe find out why the internet thinks Sherlock Holmes is a fucking arsehole. "Yeah, all right," John agrees, and he follows Sherlock out the door and into a cab.

"So this is what you do?" John asks in the cab. "Help the Muggle police? Go to crime scenes and do your…whatever it is?"

"Deduction," Sherlock says. He hasn't looked up from the screen of his mobile since they got in the cab.

"The science of deduction," John mutters, and Sherlock glances at him.

"You saw the website," he says.

"Yes," John admits. "It sounded kind of ridiculous. But…" But then you explained how you dissected my life, he doesn't add.

The crime scene is a small home on a quiet residential street, with police tape strung across the gate to the small, fenced-in yard like a grim party streamer. Uniformed officers keep out curious passers-by, but Sherlock and John are ignored as they slip between the patrol cars parked out front and through the gate. They aren't challenged until they reach the front door.

"What the hell are _you_ doing here?" sneers a weedy man with elaborately oiled hair, stepping into the doorway to block Sherlock's entry. He is wearing a baby blue jumpsuit, latex gloves, and booties over his shoes.

"I think that should be obvious even to your limited intellect," Sherlock replies. His tone is so cool and matter-of-fact that John could almost believe Sherlock is unaware that he just insulted the other man.

"I'm handling the forensics here," the man says hotly. "And I don't want you mucking about as if you've never heard of chain of custody or cross-contamination."

"I don't believe that's up to you, Anderson. And never will be," Sherlock says. He steps toward the door but stops when Anderson doesn't move; evidently he's hesitant to resort to physical force.

"Hey, Donovan," Anderson says, turning slightly to call back into the house. "Did the DI invite the psychopath over again, or is he blowing smoke up my arse?"

"Sociopath, Anderson, and please don't make me consider your arse." The sardonic tone causes an involuntary smile that John tries to hide behind his hand. 

"Unfortunately," says Donovan, appearing behind Anderson. "Sherlock Holmes plus guest. Apparently we're hosting mixers now, not solving crimes." Donovan is a slender black woman, also wearing a blue jumpsuit and an extremely irritated expression.

"Don't delude yourself, Donovan, you veryrarelysolve any crimes." Sherlock flicks his fingers at Anderson in a shooing motion, and he grudgingly backs out of the doorway so that Sherlock and John can enter.

Anderson turns his glare on John. "I've given up tellinghim, but you're going to wear a suit," he orders, pointing at a couple of open plastic crates set to one side of the front door.

John shrugs and finds himself a plastic-wrapped jumpsuit and a pair of flimsy booties. Sherlock contents himself with a pair of latex gloves he grabs from a half-empty cardboard box. John is just pulling on his own when Lestrade comes out of the sitting room to their right. "Ready?" he asks. "It's back through here."

John trails Lestrade and Sherlock through the sitting room to a large eat-in kitchen, whose most interesting feature is currently a man's body slumped against the cabinets by the sink. The man is slightly heavy but not fat, middle-aged, and dressed in a cheap suit. 

Sherlock runs his eyes over the corpse, up and then down, walks over to it, and stoops to peer at the cabinet it's leaning against. He stands and looks down into the sink, frowning. "This tap was on when the body was found. Who turned it off?"

"It wasn't on when we got here," Lestrade says. "The neighbor, maybe. He's the one that called it in."

Sherlock makes an irritated noise and goes back to the body. He pulls a small magnifying glass from his coat pocket and examines the hands and face of the corpse, then the feet and ankles. "Doctor Watson, what do you think?" he asks suddenly.

John glances at Lestrade, who makes a hissing noise as he exhales sharply, but doesn't say anything. John goes to kneel on the opposite side of the body from Sherlock, wincing at the pressure on his leg. He leans the cane against the cabinet next to the corpse and looks at Sherlock, who just watches him placidly, not giving any sign or clue of what exactly he's looking for.

John inspects the hands but doesn't see what Sherlock was looking at. He raises one arm, finding it surprisingly easy; he expected the corpse to be much stiffer. He lifts the calf slightly- again, easier than it should be- pulling up the leg of the man's trousers to examine the skin where it had been pressed against the floor. He pulls up one of his sleeves to examine the forearm. There are flecks of dried blood on the corpse's lips, and a small trail at the corner of the mouth. More spots on the collar. The man's jacket is already open, so John untucks his shirt, unbuttons it, and spreads it open so he can see the corpse's chest and abdomen.

The stomach is slightly more rigid to the touch than the arms and legs. It is also darker than the surrounding skin, with deep purpling toward the center. "Huh," John says, half to himself. The pattern is familiar. Hey, wait, he says aloud, and looks up at Sherlock. This is very similar to those photographs you showed me yesterday. Was that one of the previous victims?

Sherlock, have you even _heard_ of confidentiality? Lestrade demands. He squeezes one hand to a fist in his hair and looks about to yank out a fistful.

Sherlock, oblivious to Lestrade's obvious anger, just waves a hand dismissively. I don't want to bias your thinking. Carry on.

John puts aside his memory of the photographs and Sherlock's questions in the lab the previous day, and views the corpse as if it is the first of its kind. He runs his hands over the abdomen and chest to supplement his visual inspection, but doesn't feel any sign of laceration or abrasion. He flips the top back together and starts to re-button it.

"Internal bleeding," he says decisively. "Something abdominal, obviously. Could be any number of causes- cancer, an ulcer, vitamin K deficiency."

"Blunt force trauma," Lestrade suggests.

"Not likely," John says.

"What about the bruising on the stomach?" Lestrade asks. He's leaning against the kitchen table, watching John with a guarded expression.

"Not bruising, in the sense that you mean. The discoloration is caused by blood pooling under the skin, but it covers the whole abdomen. It's too broad and non-specific to be the result of a beating, John explains, largely for Lestrade's benefit because it's essentially the same analysis he gave Sherlock yesterday in the lab. He uses the cane and then the counter to push himself upright.

"Time of death?" Sherlock asks.

"Twenty-four to forty-eight hours," John says at once. "It's hard to be precise, but definitely more than a day." This is kind of a rush. John wonders if the Met has an opening for a consulting doctor.

"The pathologist said less than three hours," Lestrade objects, neatly bursting John's bubble. "Victim is Christopher Ames, of Consolidated Chemical's London sales office. He was at work until six yesterday, but didn't show up this morning. The office called his emergency contact, who came into the house and found the body."

"Oh." John can feel his face flush with embarrassment. It probably was ridiculous for him to think that he could walk into a crime scene and figure things out the way a trained investigator could. Perhaps Sherlock's confidence was just catching.

"He wouldn't be displaying that level of lividity if he'd been there less than three hours," Sherlock says. John blinks slowly, because Sherlock is right. And if he knows that much about time of death estimation, why the hell did he bring John here? Why did he actually seem to be interested in John's ideas while he'd been talking?

Sherlock returns to the arm John was just looking at and pushes the sleeve back up. "Where's his watch?" he asks.

"What watch?" Lestrade asks.

"Precisely," Sherlock says. "There's an imprint from the engraving on the back of his wrist. He wears it every day, so he wouldn't get ready for work without putting it on. If you don't know about it, that means it was gone when you got here."

"The killer took it," John says.

"Yes," Sherlock says simply. He turns out the corpse's pockets and finds a wallet, which he opens and rifles before passing it to Lestrade. He goes to a shallow dish at the end of the counter, from which he plucks a keyring. A moment's work with the magnifying glass and he tosses the keys to Lestrade as well.

"Well? Let's have it," Lestrade says.

"Obviously the victim was murdered, the scene rules out accidental death. If the previous deaths are in fact as similar as you tell me, I would speculate that they are murders as well. Same pattern, I take it? Victims found alone in their homes, dead of internal bleeding with no obvious injury, found after failing to turn up to work?" Sherlock doesn't pause for an answer before continuing, "This one died the night before last, not this morning. Ventilation took care of the odor. The only question is, what did a secretary like Ames have access to in the course of his employment that was worth stealing?"

"How do you get traumatic death? Why not natural causes?" John asks. Sherlock's face lights up at the chance to explain.

"Look at the cabinet, look at the tap! It has a single lever, and the lip of the drain is still wet- the water was running, recently. The neighbor wouldn't have had a reason to turn the tap on, nor the police, so it was already on. Why? Look at the positioning of the body, the way it's slumped over. Obviously Ames was thrown up against the counter, inadvertently pushing the tap on, then slid down to the floor.

"The wallet is still on the body and full of money, so clearly not a simple robbery despite the missing watch. So what else is gone? There's a key missing- a small one, you can see the marks where the teeth scratched the other keys on the ring. Too small for a car or house key; about right for a safe or a filing cabinet. He's also missing any form of work identification card, despite the fact that he has a clip for one attached to his breast pocket."

John's jaw almost drops. "That's incredible," he says. Sherlock smiles very faintly.

"Lovely," Lestrade says. "Only it doesn't give us any idea who the killer is, or what's making him target his victims. Or how he caused massive internal bleeding without apparently laying a bloody hand on them."

"The murderer's method is irrelevant," Sherlock says breezily. "The victims' employers are the key. Who were the first two, and who did they work for?"

Lestrade pulls a small notebook out of his coat pocket and flips rapidly through it. "First victim was Jared Hsu; he worked at the Petrochemicals Limited factory. Second victim was Rachel Barstow; she worked at the London branch office of Wainwright Forth."

"Also part of the chemical industry," Sherlock supplies. "I need to know what they did for their employers, what they had access to that someone might want. Industrial secrets, financial records, it will be something that could have a marketable value."

Lestrade hesitates for a moment, then nods grudgingly. "All right, I'll have someone make the calls."

Just then, a pair of emergency service technicians wheel in a gurney draped with an empty body bag. "Ah, good," Sherlock says. "Where are you taking it? Bart's? Excellent."

He breezes out the doorway, with Lestrade calling after him as if he has just thought of it, "Oi! Sherlock! The guy went to work yesterday, so he obviously wasn't dead the night before last!"

John limps after Sherlock as fast as he can manage. He pauses to remove the jumpsuit just before the front door, and Donovan wanders up to him while he's doing it. "You better hurry," she says. "He'll leave you here."

"How did he find you, anyway? Personal ad? Stalking?" Anderson puts in from the sitting room doorway. "Take my advice, you see him hanging about outside your flat at night, dial 999. He's mad, you know."

"He's bloody certifiable," Donovan says. John looks at her face closely, but he can't find any evidence that she's having him on. She seems very earnest, they both do. But then, it's clear from their earlier conversation with Sherlock that there's no love lost between them. So who cares what they think?

"I think I can judge for myself, thanks," John says.

They're right about one thing, though; when John gets back to the street outside, he's barely in time to dive into the cab Sherlock has hailed before it zips off. "Were you going to leave me here?" he demands.

Sherlock shrugs nonchalantly. "You know where I'm going."

"No I bloody well don't!"

Sherlock gives him a look of disdain and a raised eyebrow. "Bart's, John. I'm not sure how I could have made it more obvious."

"You could have told- oh, never mind," John says. "What do you need at Bart's?"

"To examine the victim's body. Privately." Sherlock makes a little humming noise to himself. "Curious, though. A dead man appearing at his job after he's been killed. It's nice to have a murderer who's interesting for a change. I wonder if the time of death given for the previous victims' was similarly wrong."

"So you're assuming the pathologist was wrong," John says.

"I'm not assuming anything," Sherlock says irritably. "I am building a complete picture of the situation. One that includes _all_ the evidence, not just that which can be easily twisted to suit a pet theory. That method is what causes the Yard to go wrong so often. And hence, to need me."

* * *

Their cab arrives at the morgue just ahead of the corpse, and Sherlock leads John inside while the body is signed over to the hospital's custody. Sherlock pushes straight into a small office with a nameplate reading _Dr. Molly Hooper, Pathology_ and begins haranguing the mousy woman at the desk inside. "What were the causes of death for Jared Hsu and Rachel Barstow?" he barks, without an introduction of any kind.

Hooper she simply blinks a few times before answering. "Non-specific internal bleeding as to both," she says. "I remember those because they were so odd. Something made a complete hash of their abdominal cavities but I couldn't narrow down a source for the damage."

"And time of death?" Sherlock demands.

"Also strange," she says. "Everything about the bodies suggested they'd been dead for one to two days, but they had both been seen alive within the previous twenty-four hours. Suggesting that environmental factors led to premature degradation of the bodies." She brushes her hair out of her eyes and sets aside the report she's been holding in one hand while talking to Sherlock. "I told the police they might be connected, very strange pair of cases."

"Three, not a pair," Sherlock says. "Your third corpse is being checked in now."

Hooper stands up, looking eager. "Well the third time's a charm," she says. "Maybe this one will give me a clue about the cause of death." She smiles warmly at John but her eyes are distant as she leaves her office and heads straight for the main room to take charge of the corpse. Sherlock follows swiftly, and John follows after him.

"I need you to distract Molly," Sherlock says under his breath, while the doctor is directing the techs where to put the body bag.

"The pathologist?" John asks, equally quiet. "What do you mean, distract her?"

"Yes. Get her out of here, I need a minute alone with the body," Sherlock says, and walks toward the gurney as the techs depart.

"How am I supposed to do that?" John mutters, starting after.

Sherlock moves quicker than a striking snake; his foot kicks John's cane out from under him and is innocently pulled back before John even hits the floor. The stumble itself doesn't hurt, but catching himself on his thigh to avoid smacking his head on the floor causes a twinge in John's leg. His bitten-off curse is completely genuine and makes Hooper turn to him instantly.

"Oh my God, are you all right?" she exclaims, hurrying over.

"Yeah," John says. "Yeah, it's just- sometimes, the leg- you know." John is a bad liar, but under the circumstances his stammering just makes him look more pathetically helpless. He flushes with humiliation and anger.

"Can I get you anything?" John stifles a scowl at Hooper's solicitousness; she's only trying to help.

"He's fine, Molly, quit fussing," Sherlock says, staring at the body bag with naked avarice.

Molly flushes. "Don't mind him," she tells John. "Anything? Ice pack? Parecematol? Cup of tea?"

"Any and all of the above would be lovely," John manages.

"You just wait here, I'll be right back," Hooper promises, and rushes out the door, letting it swing shut behind her. Sherlock is unzipping the body bag before the click of her footsteps has faded.

John rolls upright. "You bastard! You tripped me!" he snarls.

"Well spotted," Sherlock says. He is busy tucking the edges of the body bag under the corpse's arms, exposing its chest and stomach.

"That's- you are unbelievable," John says. He is angry, but not as angry as he probably should be.

"You don't actually need the cane," Sherlock says. "And I wouldn't have succeeded in tripping you if you used it correctly." He drapes his coat over another gurney and pulls out his wand. "We should have at least three minutes. Keep an eye out."

"What are you doing?" John asks, forgetting his irritation in alarm.

"Gathering data." Sherlock appears to be tracing an outline in the air above the corpse, using the tip of his wand. He takes two steps back, makes a tight, controlled gesture with the wand, and utters, " _Ostendo_."

The result is immediate: a purple mist rises from the corpse's torso and forms an arc that begins just below the sternum and curves slightly down and away to a point two feet from the chest.

"The angle indicates that he was pushed back and upwards by the blow, just as I suspected." Sherlock walks around the gurney. "It's not Avada, but that didn't fit with the internal damage anyway." Sherlock draws a notebook and pen from inside his jacket and begins to jot rapid notes.

"That mist represents a spell hitting him?" John asks. He pushes himself upright with the cane, so he can walk over and get a closer look.

"Obviously," Sherlock says.

"I never knew you could do that," John says wonderingly. "See an after-image of a spell."

"Only if you know how to look for it," Sherlock says, pocketing the notebook again. "Forensic spellwork is a much under-utilized field." He flicks his wand at the purple mist, and it dissipates. He zips the body bag back up. "Back to Baker Street. Come on."

Hooper comes back in, with an ice pack in one hand and a mug of tea in the other, as Sherlock is walking through the door. John hesitates a moment, torn between basic politeness and the sure knowledge that he will be left behind if he doesn't hurry to catch up. John doesn't stop to wonder why he _cares_ about being left behind. "Sorry," he says to Hooper apologetically. "Thank you, really, but I've got to catch him before he gets outside, or he'll get a cab without me."

She bites her lip, seems on the point of saying something, then just nods. "If you're sure you're all right then," she says.

John smiles reassuringly. "I'll be fine. I'm getting used to it." He's not sure if he means the leg, or rushing after Sherlock.

Back at the flat, Sherlock goes immediately to a shelf in one of the bookcases that has already been filled with a neat row of hardbound lab books and selects one. He drops onto the sofa and flips it open, beginning to page rapidly through. John wanders over to the lab books and examines the spines; they are unlabeled, so Sherlock must have selected that one from memory, despite the fact that there are dozens of books on the shelves.

Sherlock's phone rings, and he answers it one-handed after a brief glance at the display. "What have you found?" he demands. John sidles slightly closer, almost unconsciously, as the conversation goes on, but is unable to hear whatever the other person is saying. Sherlock listens for several minutes in silence before snapping, "That's ridiculous. What's his name? I'll question him myself." A briefer pause, followed by, "Lestrade-" Sherlock removes the phone from his ear and glares at it for several seconds before dropping it back into his pocket.

"What's wrong?" John asks. "That was the detective inspector, I assume?"

"Obviously," Sherlock says scornfully. "Hsu was apparently a janitor, and naturally had access to every part of his employer's property. Barstow was a secretary, who at the time of her death was involved in a project to digitize certain business records of her employers. Ames was a secretary, as I had already deduced, but as part of his duties he had access to the secure area in which sensitive records were stored."

"What business records?" John asks, remembering Sherlock's earlier insistence that the information to which the victims had access was what the killer was after.

Sherlock clenches one fist where it lay on his thigh. "Exactly," he says. His eyes are bright with intensity, or perhaps just anger. "Evidently Ames and Barstow's superiors refused to provide that information to the police. And because Lestrade's constables are insufferable idiots, they evidently accept 'I can't tell you' as an appropriate response to police inquiries." He scowls. "When I have the information I need here, we will have to go to these managerial incompetents in person. A thorough waste of time, but I must have the proper data if I am to determine what the murderer is seeking and how best to discover his identity."

Sherlock begins to flick through the lab book in his lap again. John cranes his neck and sees pages of closely written notes, supplemented by sketches of bodies with lines and arcs extending from them. Sherlock pulls out his pocket notebook and opens it to the notes he just made at the mortuary.

"That spell at the mortuary," John says with dawning awareness. "You wrote it."

"Yes, of course." Sherlock runs his finger down a page of the lab book, then frowns and flips to the next.

"And you tested all these spells to see what they'd look like?" John remembers what Sherlock said in the lab about Avada and feels ill. "Oh my god. You tested the Killing Curse?"

"On a pig, yes," Sherlock says, looking up. John relaxes a little; Sherlock is a bit strange, but he's hadn't seemed like a killer. John feels embarrassed about jumping to conclusions, but Sherlock doesn't seem to be bothered. He is wearing a bemused smile as he looks back at the lab book. "Ah, here. Mid-range purple shade, low degree of spread, sharp arc." Sherlock's smile reverses itself and becomes a frown.

"So what is it?" John asks.

"Minuo viscera," Sherlock says. He sounds distinctly unhappy. "That's…." His voice trails off, and there's a pause before he seems to recover. "That's consistent with the injuries, at any rate. It causes massive internal bleeding."

"So the spell killed him," John says flatly. "A wizard killed him." As if there hadn't been enough of _that_ to last a lifetime over the past two decades!

"Unfortunately," Sherlock says, frowning. He rolls his eyes when he sees the expression on John's face. "Don't be so tediously predictable, John. You've been of help thus far because you have an unbiased perspective, so I'd appreciate it if you didn't start leaping to conclusions involving dark wizards."

"You have to admit it's suggestive," John says.

"I'll admit no such thing," Sherlock rejoins. "One of the myriad reasons I do not work with the Aurors is because for them, everything points back to Voldemort. It's sheer intellectual laziness and it compromises investigations." Sherlock sighs heavily. "I hate when cases involve magic, it's boring."

John can't help but bristle. "So that's what bothers you about this? Not wizard-on-Muggle murder. Just that it's not interesting enough for you?"

Sherlock raises an eyebrow. "Wizards are careless and sloppy with their murders; it comes of thinking they can solve all their problems with spells. Ames would be just as dead if a Muggle had killed him, so what possible difference does it make that the killer was-" His eyes widen and he breathes, "Oh!"

"What?" John asks.

"Shut up a minute." He has his wand out again, poking from between his flat palms, which he steeples beneath his chin. He rolls the wand slowly between his palms and lets his eyes drift shut. John just stares.

"Ah!" Sherlock's eyes pop open. "Polyjuice potion. Obvious." He looks at John as if expecting approval. John can't even manage comprehension.

"Sorry? What's obvious?"

"The victim appeared in public after his death. Come on, you grew up in a wizarding household." Sherlock looks at John's uncomprehending face and sighs gustily. " _God_ , you're as thick as the Yard _._ He kills Ames, steals his watch and his ID so he can pass as him, but he has to look like Ames to make it work. How does he do it? Polyjuice potion. Takes a month to make, so he's been planning for a while. The dosage determines the length of the transformation, he could easily create enough to disguise himself for the entire work day. When he's done, he walks out of the office and becomes himself again. Only now he has the information he wanted which Ames had access to."

"What information?" John asks.

" _Exactly_." Sherlock sits back. "But the methodology is obvious, once you realize magic is involved. I told you wizards are sloppy. But you should be happy, it completely vindicates your evaluation of the cause and time of death."

The mobile rings again, and Sherlock answers. "Lestrade, I presume you've reconsidered?" he snaps. As he listens to the response, his eyes immediately light with interest. "No," he says. "That isn't right." A pause. "Which chemical plant? Did he go to work last night?" Another pause. "Of course. I'll see you in a few minutes." He's grabbed his coat and is flying down the stairs in seconds, leaving John running to keep up. The cab is pulling over as John closes the door to 221 behind him, and he enters in time to hear "47 Bowditch," barked at the driver.

"What's happening?" John asks, settling back in the seat. Clearly it's not just the information about the documents, Sherlock seems far too excited for that.

"A fourth! Gerald Halbrud, a night security guard at Chemeuro's London manufacturing plant, was just found dead in his home, in the same manner as our previous victims," Sherlock says. 

"That's awful quick work," John says slowly.

"Exactly!" Sherlock practically crows. "It's far, far too soon for another kill." His eyes are gleaming, and he begins to mutter furiously. "Second body found two days after first, third body found two days after second. Fourth body found four hours after third, no day of work between murder and discovery. Pattern broken. Intentional? No. Accidental? Likely. Why? Ahh." Sherlock smiles, a quick flash of a grin that immediately disappears again. "Someone found the body, someone whose arrival had not been expected. A girlfriend."

"How do you get that?" John asks.

"The other victims were all unattached, which meant that the murderer did not have to concern himself with the risk that their bodies would be found until after they had been missed at work. He had the leeway to murder each victim, leave the body where it fell, and go to work in the person's place on the next business day. But this time, the killer didn't have the choice of access points that he would like, and was forced to choose Halbrud, who had a girlfriend with keys to his flat. The body was discovered before the killer intended, because the girlfriend returned unexpectedly from a trip. He's made a _mistake_!" Sherlock's excitement over this murder is almost indecent, but John can't judge him too harshly because it's also contagious.

"A mistake, that's significant?" he asks.

"Clearly!" But Sherlock is obviously too thrilled to be properly disdainful. "We have him, John! We'll examine the scene, and then we'll catch the murderer when he makes his attempt to steal from Chemeuro, disguised as Halbrud."

They tumble out of the cab at a block of flats lit by street lamps and the flashing lights of patrol cars. Lestrade is standing on the edge of the lit area, frowning and hunched in his overcoat. "Something's wrong," Sherlock says immediately upon spotting him, and strides over so quickly that John has to jog to keep up. "Why aren't you inside?" Sherlock demands. "I expect that the scene will reveal nothing new about the killer's methodology, but I should at least inspect it to be sure. Which flat?"

"We can't," Lestrade spits out. "You're out, Sherlock." Sherlock's face darkens, but before he can speak Lestrade continues, " _We're_ out. My whole team's off the case."

"What?" Sherlock says. "This is- Lestrade, you are being absurd. The case is three quarters solved. We know in general terms why the murders are happening, all we need do is-"

Lestrade makes a cutting motion with one hand. "Look, I don't like it any more than you do," Lestrade says harshly. "But the Home Secretary called my commander, and apparently there's some national security issue, and- well. It's not a matter for the Met any more, or so I'm told." He scowls. "Regardless of how many hours my team has put in."

"Whose matter _is_ it then?" Sherlock asks. His eyes are scanning the building behind Lestrade. "I'll talk to whoever it is. They'll need me as much as you did."

"Sherlock, they specifically said that you were barred," Lestrade says.

John decides to intervene before Sherlock actually vibrates to pieces with rage. "Who's they?" he asks reasonably.

Lestrade darts a surprised glance at him, as if he's just now noticed John is present. "Agents Howard Dye and, uh- something Perryvall."

" _Cassius_ Perryvall?" Sherlock's voice is incredulous.

"That might have been it," Lestrade says. "Why, do you- Sherlock! Hey!"

Sherlock is striding furiously toward the block of flats and a tall, dark-skinned man in a somber charcoal suit and wingtips who is standing beside it speaking to a uniformed police officer. John ducks past Lestrade and follows on Sherlock's heels, because damn if he's going to be left standing on his own by the cordon, looking foolish and out of place. At least if he's with Sherlock, the man's flamboyant presence distracts people from even noticing John is there.

Halfway to the building, John notices another man- also in a suit, a hair shorter than the other, years younger, padded around the middle but still fit enough- moving on an intercept course. "Sherlock, incoming," John calls softly, and Sherlock hesitates, turning his head in the indicated direction, which allows the man to get in front of them. The tall guy by the building has looked up at them and is also headed their way.

"Out, Holmes," the young man barks.

"On whose authority?" Sherlock sneers. "Hardly _yours_ , Dye, when they still have you trailing more experienced agents like a confused duckling."

"The bleeding Minister's, you blasted-"

A long, elegant hand on Dye's shoulder cuts him short. "That will do, Dye." The tall man placidly appraises Sherlock and John.

Lestrade came running up behind Sherlock and John. "Sherlock, I swear, if you get me suspended again, I'll-"

"Detective Inspector, thank you for your diligence, but I assure you that no one in my office will hold you the least bit responsible for any action of Sherlock Holmes'." The man's short, black hair is shading into grey, and his eyes are piercing as they flit from Lestrade to John. "Would you mind excusing us? I'd like to explain the situation to Mr. Holmes personally." Lestrade backs off, obviously relieved to have _this_ particular duty shifted to someone else. John can't blame him; if Sherlock mildly irritated is cutting and nasty, Sherlock in a rage must be terrifying. John would like to see it, actually.

"You too," Dye says to John, as Lestrade retreats back to the cordon.

"No," Sherlock says before John can react. "He's fine. He stays."

"Are you sure?" Dye says doubtfully, still eyeballing John.

"Exposing you idiots to Muggle London would be as annoying to me as to everyone else, much as I would enjoy your resulting panic," Sherlock snaps. "Of course I'm sure."

"Oh, you're with the Ministry," John says, suddenly realizing why Perryvall had cut Dye off so abruptly and then sent Lestrade away. "LEP?"

Dye visibly bristles at the reference to the Magical Law Enforcement Patrol, and Sherlock laughs. "Auror Office," the other man says. "Cassius Perryvall, Senior Auror." He tilts his head to indicate Dye, removing his hand from his shoulder at the same time. "Howard Dye, my partner." He turns his attention back to Sherlock without waiting for John to introduce himself. The Aurors are the best of the best, in terms of magical law enforcement; mistaking an Auror for a member of the LEP is somewhat like mistaking an MI6 agent for an infantryman. "How long did you think you could keep this a secret from the Ministry, Holmes?" Perryvall continues.

"It's not a matter of keeping it a secret, it's a matter of you being too unobservant to realize that wizards might be involved," Sherlock says. "When _did_ you notice?" Sherlock watches Perryvall's face carefully. "Oh, yes, within the past hour. Typical."

"I'm not playing these games with you, Holmes," Perryvall says. "A wizard has been killing Muggles, which means this is my case and my crime scene. And I don't want you here. If the Muggle police knew you like I do, they wouldn't want you on their cases either."

"Wouldn't they, Sherlock says curtly. John glances quickly between the two wizards. Interesting-does this have something to do with why Sherlock handicaps himself, solving Muggle crimes instead of working with magical law enforcement at the Ministry?

"We've worked out what spell the killer used," Perryvall says. "And interestingly enough, it's-"

"Antonin Dolohov's spell, I know," Sherlock says. John stifles his sudden intake of breath, and flushes as Dye hears and glances sideways at him. John knows Dolohov's name: he was one of Voldemort's inner circle, had escaped Azkaban twice and murdered at least three members of the famous Order of the Phoenix.

"Two steps ahead as usual?" Dye asks sourly, curling his lip

"You'd need seven league boots to get within two steps of me, Dye," Sherlock snaps.

"It was his trademark during the second war," Perryvall goes on, locking Sherlock in his unwavering gaze. 

"Dolohov is in Azkaban," Sherlock says. Used to be that if you once escaped Azkaban and they caught you, they let the Dementors- John decides he is not going to think about that, not right now.

"Yes," Perryvall agrees, leaning slightly forward. "That spell only ever appeared in writing once, Holmes. How many people do you imagine know how to cast it?" The tone of Perryvall's voice is almost threatening, but John isn't sure how this is meant to intimidate Sherlock.

Sherlock seems more irritated than intimidated in any case. "You are wrong," he says fiercely. "You are a fool, Perryvall, so caught up in what you think you know that you can't observe the evidence that's directly before you."

"Get off my crime scene," Perryvall suggests, stepping back after a long look at Sherlock. He turns and walks back toward the building, abruptly dismissing Sherlock from his attention.

Sherlock spins on his heel and stalks back towards Lestrade. "Come on, John."

Dye catches John by the arm as he turns to follow, and mutters, "I don't know who you are, mate, but take it from me. You want to keep well clear of Sherlock Holmes. Be sorry if you don't."

John jerks his arm free and refuses to dignify that with a response. He catches up with Sherlock, who has stopped to talk briefly with Lestrade. "Incompetents," he's ranting to the detective. "Utter incompetents, both of them."

"Perryvall seemed pretty sharp," John says, and Sherlock turns to glare at him.

"I would not trust Cassius Perryvall to investigate the demise of a pigeon," Sherlock says bitterly. "Lestrade, listen to me and you can have the killer tonight, I promise you."

"Sherlock, it's not my investigation any more," Lestrade says. "And if the new boss doesn't want your help, there's nothing I can do about it."

Sherlock turns his back and stalks down to the road without another word to Lestrade, and John hurries after. "We'll have to walk up to the high street to get a cab," Sherlock says, turning that way. He has his mobile out and is furiously tapping at the keys.

"All right," John says amiably. "Where are we going then?"

"The Chemeuro plant where Gerald Halbrud worked as a security guard," Sherlock says. "I'm getting the address now. I've determined based on the early discovery of the body and the fact that Halbrud was not scheduled to work last night that our killer killed Halbrud this morning and expected to turn up for his shift this evening. I can see the discovery of the body has not yet been reported in the news media, so the killer should be unaware and therefore has no reason to alter his plans. I do not know the exact time Halbrud is scheduled to begin work, so our best chance of apprehending him is to go directly to the factory now." John stops walking, and Sherlock takes three more steps before turning back and looking up from his phone. "Problem?" he asks.

"You're a civilian who's just been ordered by both Scotland Yard and the Ministry of Magic to keep away from this case, and your response is to go confront a serial murderer on your own?" John asks.

Sherlock raises one eyebrow. "Yes," he says, drawing the word out.

"That is utterly, barking mad," John says. His throat has gone dry, but he finds he's not feeling apprehensive at all.

"Does that mean you're not coming?" Sherlock asks.

John swallows. "I didn't say that," he says. "I just wanted to be sure we're on the same page." It's clear at this point that Sherlock and sanity are _not_ on the same page, perhaps not even in the same book. And John is excited about this stupid plan, so what does that say about _his_ metaphysical location?

"All right then," Sherlock says. He puts out his hand and a passing cab pulls over almost immediately. John makes a mental note to ask if Sherlock is using magic to summon taxis, as they always appear when he wants one.

"If we're doing this," John says, "I need to stop back at my flat first."

Sherlock looks him over carefully and says, "Is it important?"

Sherlock didn't respond with a gusty sigh or a roll of the eyes, to John's amazement. Does he suspect what John has at the flat that's worth taking time to go back for? It wouldn't surprise him, but he hopes not. He hopes Sherlock isn't going to roll off a chain of deductions that ends with _So obviously you keep an extremely illegal firearm in your desk next to your laptop_. He tries not to let this concern show in his face as he just answers, "Yes."

Sherlock steps back from the door of the cab. "You take this cab then. Meet me at the factory." He recites the address.

"Wait for me," John says, getting in. "I know I haven't been that helpful to you, but I do know my way around a fight. I don't like to think of you confronting a multiple murderer all on your own."

Sherlock nods and slams the door shut, then raises his hand to summon another cab.

It's a detour of perhaps ten minutes to drop by John's flat, pay the cabbie extra to wait, and dash up to the flat for the Browning. John's hands are perfectly steady as he tucks it into the back of his waistband, carefully hidden under the baggy jumper. Then back to the cab, where he balances perfectly on the edge of the seat, fingers shaped into a cradle around his knee. The chime of a new text message pulls him out of his reverie.

 **Message received**  
Far too slow. He's here. Going in. SH

His first thought is, _Oh shit._ Clumsily, he thumbs out a reply on the keyboard: "Wait for me damn it I'm almost there. John"

There isn't any reply, and John's hand is clenched in a fist on the door handle by the time the cab pulls up outside the factory, his other hand digging into his thigh to avoid reaching for the gun. The factory is a massive block of a building apparently designed before the advent of windows. John's eyes scan the side and find a metal fire door which is propped open and leaking light from a concrete stairwell. All the inner doors are locked. He pauses a moment to send another text: "I'm here where are you?" but again there's no answer. John begins to climb.

On the fourth floor, he finally finds a door that opens into the building proper. There's been no sound, no sign of Sherlock, the killer, or anyone else, and no new texts. John can only be five minutes behind Sherlock, so where the hell did he go? John grits his teeth and reassures himself with the thought of the gun pressed against the small of his back. Maybe Sherlock is secretly a master duelist, but John tends to doubt it. Most wizards aren't up to facing a duelist fighting to kill.

The fourth floor door lets John out onto a catwalk that runs around three sides of a massive, open factory floor. There's machinery here and there, some of it looming up several stories, and what appear to be conveyers, down on the floor. Distantly, across the room and on the ground floor itself, two men are grappling. At this distance, John can make out that one of them is tall and wearing a long coat, one is stouter and shorter. John runs.

Somehow he resists the urge to yell out, putting all his breath into motion: finding the nearest staircase, descending the stairs two at a time, running along the catwalk to find the next. In his half-second glances as he rounds corners and leaps down stairs, John can see that Sherlock and the other man have their fists twisted in each other's shirts, straining against each other like wrestlers. John pauses half a second as he hits ground to pull the Browning out of his waistband, then pounds towards the struggling figures.

Sherlock is doing his best to choke his opponent with his own collar; the man is in turn squeezing Sherlock's wrist with one hand and trying to club him in the side of the head with the other. John's run is eating ground but he's still too damn far away when the man- the killer, surely?- manages to hook one foot behind Sherlock's ankle. In a second he has Sherlock down on the ground and is staggering back, pulling his wand up and back as if he will swing it like a sword, beginning to speak: "Mi-"

John slams himself to a halt and raises his gun. _Time to make a command decision, Watson._

The bullet punches through the killer's upraised arm and into the right side of his chest. John runs to the man, flips the safety back on and sets the gun aside as he kneels to rip his shirt open. The wound is already gushing blood, and John can hear the man's raspy, wet breathing and the sucking gasp of air in his chest. The bullet apparently was lucky or unlucky enough to catch the lung. He rips off his own jumper and presses it against the wound, trying to staunch the bleeding even though he knows that's virtually impossible.

"Call an ambulance," he snaps to Sherlock, who has risen to his feet. 

"No signal," Sherlock replies instantly. "This building was an air raid shelter." John doesn't point out that the simplest way around that is to _go outside_. Surely he can bloody figure it out, and John has work to do. He tunes Sherlock out completely.

John doesn't have anything to improvise a seal with, although it may be irrelevant because the man's going to drown in his own blood before he really has time for the pneumothorax to become an issue. John finds the artery nearest the wound and tries to put pressure on that, but it's equally infective. Then breathing stops; John opens the man's mouth to attempt rescue breathing but his mouth and throat are fullof blood. "Shit," John mutters, as he feels for a pulse and doesn't find one. He starts hands-only chest compressions, because that's what you _do_ ; and anyway, it feels somewhat unsatisfying to apprehend a killer by shooting him instead of arresting him.

Two minutes and at least six cracked ribs later, John gives it up and staggers back from the man- body- with blood splashed across the front of his jeans and saturating the sleeves of his shirt. He sits down heavily on the grimy concrete, only now starting to breathe hard. His left hand, the shaky one, is still rock steady though. Odd.

Sherlock is standing well back from the pool of blood, wand in one hand, staring at John.

"Oh, what _?_ " John demands, feeling a flare of annoyance; irrational, surely, since it's probably normal to stare when someone guns down a man in cold blood right in front of you.

"You killed him," Sherlock says. "Why?" John is having a hard time reading the expression on his face; is it fascination, or shock, or horror, or what?

"Because he was pointing his wand at you," John says. "I told you to wait for me, you _tosser_."

"You didn't even know what he was going to cast," Sherlock says. He ignores the second statement entirely, which really isn't a surprise. It seems to be Sherlock's modus operandi when dealing with common sense: just ignore it and perhaps it will go away.

"No, I didn't," John agrees. This should probably bother him, but he finds he's completely comfortable with it. Christ, what's wrong with him?

"You barely know me," Sherlock points out. His tone is business-like, not shocked or horrified at all. 

"Also true." John reaches for the gun and realizes his hands are coated in blood. He wipes them clean on the hips of his jeans as best he can.

"Incredible," Sherlock says. Definitely fascination, John decides. God, they're _both_ twisted.

"Glad you approve," John says, finding that this is actually true- Sherlock's open appreciation inspires a warm little spark of pleasure to flare deep in his chest. "Just out of curiosity, this is the killer, right? Only I'd hate to think I murdered some poor night watchman who was only trying to keep a dangerous lunatic out of his factory."

"Definitely the killer," Sherlock says. "He's carrying Halbrud's identification card, and wearing his face."

"Brilliant," John says. "So when does he change back to his own?"

Sherlock gives John a pitying look, as if he's said something quite daft. "If someone dies under the influence of polyjuice potion, the effect is permanent."

John takes a second to parse this. It means that the police, the Aurors, have no way of knowing for sure that this isn't Halbrud. It means that they can't identify this man as the murderer, or identify him at all. It means that John is going to be in rather a lot of trouble, very shortly. "Oh," John says, staggered by the weight of the realization. "Oh, fuck me."

Sherlock stalks over to the body and retrieves John's crumpled, blood-soaked jumper with the tips of his fingertips. He holds it out away from himself with one hand, and aims his wand at it with the other. " _Scourgify_ ," he casts, and the blood clears itself from the jumper until it's the proper oatmeal color again. Sherlock flicks the wand twice more, giving John's other clothes the same treatment. 

"What are you doing?" John asks, watching the blood disappear from his jeans; in the Army they washed out blood the old-fashioned way, and he's forgotten how easy magic makes some things.

"Destroying evidence, obviously," Sherlock says testily. "Put out your hands." John obeys. " _Tergio_." He siphons the blood off John's hands and sends it back to the body. "Now we can get a cab."

"But shouldn't we call-" John pauses for a second. If Sherlock isn't feeling inclined to call the authorities, why the hell should John? "You know what, never mind. Let's just go." 

Sherlock smirks very slightly. "Knew you'd see things my way," he says.

John scoops up the gun with his newly-clean hands and tucks it back into his waistband. Sherlock tosses him the jumper as he stands up, and John puts that back on too.

They're getting in the cab when Sherlock looks at him and says innocently, "Where's your cane?"

His _cane_ , he must have left it in the factory- no, back up, he had been running inside the factory, not using the cane. Did he leave it in the cab? No. He left it-

He left it at Baker Street.

John's leg feels fine. He settles himself in his seat and starts laughing, helplessly. "It _is_ psychosomatic," he gasps when he's able to stop giggling. John relaxes a little more, rolling his shoulders. He looks over and sees Sherlock giving him that very intrigued look again. "What?"

"Are you all right?" Sherlock says carefully. "You seem- very calm. For someone who's just shot a man, I mean."

"Shht!" John hisses, darting a glance up at the cabbie, who fortunately seems oblivious. "Have a lot of experience with killers, do you?"

"Well, yes, but typically they are rather unbalanced." Sherlock considers John for a moment. "Perhaps this is how a balanced person acts when he kills someone."

For some reason, whether it's the words themselves or Sherlock's calm detachment as he says them, John finds this utterly hilarious and bursts out laughing. Sherlock joins him, this time, his mouth spread in a smile that somehow looks deeply genuine, not like the image of a smile that John's seen him plaster on all day.

The MOD flat to which John returns in the small hours seems colder and emptier than ever, but somehow John can't bring himself to mind. The adrenaline burned out of his system over risotto and wine at the restaurant Sherlock dragged him to, and now he's tired. John sets his Browning down on the desk and does the evening routine: uses the loo, brushes his teeth, changes his clothes, turns the sheets down. Tonight he adds an extra few minutes scrubbing his hands clean before he fetches his cleaning kit out of the desk. John removes the magazine and field strips the gun, laying the parts neatly in a row. Each part is cleaned and oiled, then laid back on the desk. Last of all he oils the barrel, then slowly and carefully reassembles the gun and reinserts the magazine. It doesn't have to be slow, of course, but John likes it to be. 

The routine is always soothing, but tonight it takes on a special importance. It is after all the first time he's fired it. When he is done, he carefully caps the oil, bins the used patches, and tidies away all the supplies into the desk. It occurs to John that he should be freaking out. He should be worried about the fact that he just shot a man in cold blood; that the act was witnessed by a man who styles himself a detective and consults for the police; that the police are going to be after him; that the gun he shouldn't have in the first place is now a murder weapon. But he isn't. This may in fact be the least worried he's felt since before he got shot.

Every night since he bought the gun, John has felt compelled to flick the safety off and stare at the weapon. Sometimes he has to pick up the gun and twist his wrist so he can press the end of the barrel against the edge of his hard palate, carefully maintaining the proper angle. Many nights he has closed his eyes and thought about the cold, hard bite of unyielding metal cutting the roof of his mouth, the taste of oil and steel heavy on his tongue; he has imagined the tension dripping with the blood and cerebrospinal fluid out of an exit wound in his parietal bone.

Tonight, John puts the gun back in its drawer the instant he finishes cleaning it, without a second thought.

John still feels oddly good when he wakes up in the morning. It lasts through breakfast and getting dressed, till about 9:30, which is when his phone rings.

"Those Aurors are here," says Sherlock Holmes.

John's stomach clenches. "Are they?" he says carefully.

"They want to speak to us. Both of us, apparently," Sherlock says, just as calmly.

"I'll take a cab then, shall I?" John asks.

"Yes." Sherlock hangs up.

John's stomach is knotted with tension through the entire trip, although he maintains his facade of calm. They must have found something at the factory to link him or Sherlock to the scene. Or else- what if Sherlock called them? Surely not, he had made his feelings about the Aurors very clear. But he is a man who persists in solving crimes even when he's been specifically told not to, and he obviously loves to prove he's clever. Not calling the police from the factory last night might have been simple self-preservation. It would be easy enough for Sherlock to distance himself now, and all they need do is search John's flat for the gun….

When John knocks at the door of 221, it's Perryvall, looking stern and official in black robes, that meets him at the door and escorts him up to the flat. Mrs. Hudson is hovering in the hallway, fluttering her hands nervously; John smiles at her, but she doesn't follow them up to Sherlock's rooms. Dye stands just inside the sitting room, looking grim, but Sherlock lounges unconcernedly on the sofa with his hands steepled under his chin, wand clutched between them. He sits up as John and Perryvall walk in. "Very well, here he is. Can you stop wasting my time now, and tell me what this is all about?"

"Where were you after you left Halbrud's building last night?" Perryvall asks, crossing his arms over his chest.

"We went to Angelo's Trattoria for a late dinner," Sherlock says.

"At nine thirty in the evening?" Dye says skeptically.

"The owner is a friend of mine. He stayed open late for us," Sherlock says. John has to stifle the urge to look at him admiringly; the man lies beautifully, and Angelo will doubtless be more than happy to provide an alibi if asked. It was clear from the way he behaved last night that he practically worships Sherlock.

"And you- what time did you finish at the restaurant, then?" Dye asks, turning to John.

"How the ruddy hell should I know? I wasn't staring at the clock the whole time," John says. "Besides which, I was drunk. We split two and a half bottles of wine between the three of us." This is actually true, which may be why John can say it without a flicker of hesitation.

"Is this inquiry into our personal lives a prelude to an invitation?" Sherlock asks. "I can't speak for Dr. Watson, but I can assure you that I am _far_ from interested, Dye."

While Dye splutters, Perryvall cuts in, "Something interesting happened this morning, Holmes. It seems that when Halbrud's employers opened for business, they found a body on the factory floor." Perryvall whips a photograph out of his robe and flips it onto the coffee table. "What do you make of this?"

"Gunshot wound to the chest," Sherlock says, examining the photo. "It appears to be something that Scotland Yard ought to be investigating, not the Auror Office." He offers the picture to John, who walks over and takes it out of his hand.

It's a regular Muggle photograph, of the Halbrud double's corpse at the factory. "Nasty," John says, handing the photo back to Perryvall.

"Quite," Perryvall says. "That's Gerald Halbrud."

"Oh?" Sherlock says. "Seems quite out of character for our killer, wouldn't you say?"

"Or rather," Dye puts in, "That's the _second_ Gerald Halbrud. The original one was found dead in his flat last night, as you are aware."

Perryvall is looking Sherlock right in the eye. "Why don't you explain that, Holmes, as you're so clever. Enlighten us." With an effort, John manages to breathe normally. Sherlock is not going to be able to resist explaining it, not when the Aurors are challenging him right to his face this way.

But Sherlock smirks. "You wanted me out, and I'm out. Don't come begging for my help just because you found the case a bit more _difficult_ than you anticipated."

Perryvall's face clouds over. "Give me your wand," he says.

"No, I don't think so," Sherlock says. He smiles pleasantly; it's the fake smile again, John is getting good at telling the difference.

"Fine," Perryvall grates. "Just put it on the table, then." Sherlock considers Perryvall for a moment, then sets his wand on the coffee table. Perryvall draws out his own wand and gestures. " _Prior Incantato_!" Faint pink wisps emerge from the end of Sherlock's wand in response to the spell, twining gracefully around each other before dissipating. It's meaningless to John, but evidently not to Perryvall. "Tergio?" he says sourly. "Doing a bit of cleaning, were we?"

Dye glances around at the decidedly untidy flat and snorts. Sherlock does not react.

Perryvall turns to glare at John. "And you?"

"He's a squib, you imbecile, he doesn't have a wand," Sherlock says scornfully. John tries not to wince and is mostly successful.

Perryvall makes a tiny moue of distaste. "The man in the factory was not killed by a wand," he says.

"Well I hardly have a _gun_ either," John says. Sherlock must be inspiring him, this is probably the best lying he's ever done. "Just what are you suggesting?" Perryvall tries to stare him down, but John is not intimidated. This entire thing, he now realizes, is an attempt to scare them. The Aurors have nothing, know nothing; they just came here because they don't like Sherlock, not because they have any evidence to link either of them to the death.

"If that's all," Sherlock says after a moment. "I think I would like you out of this flat now." He picks up his wand and twirls in around his first two fingers like a baton. Perryvall finally breaks his gaze from John's.

"Holmes," Percyvall says. "Rest assured that if I find _any_ evidence linking you to these murders, I will personally ensure that you die in Azkaban." He turns and descends the stairs regally, Dye clattering after him. A moment later, John hears the twin bangs of the two of them disapparating from the foyer.

Sherlock throws the wand back down. "Finally," he says. "I can feel my IQ dropping every moment I share a room with those idiots."

John looks at Sherlock and opens his mouth to speak. He's pretty sure he means to say something sensible like _are we ever going to know who the killer was?_ or _what the hell did you do to piss those guys off?_ or perhaps _thanks for keeping me out of prison_. Instead, he hears himself say, "I'd like to bring my things over this afternoon."

Sherlock smiles, brilliantly, and it's the real one again, a smile of genuine pleasure. "Yes," he says. "I thought you might."


	2. Chapter 2

**Notes for the Chapter:**

> Muchas gracias again to Thesardine for betaing the hell out of this.

**wizard** (`wizərd,) _noun_  
1\. a male human born with the ability to use and manipulate magic.  
2\. _Muggle usage_ a person who is very skilled in a particular field or activity: _a financial wizard_  


    

\-- _Oxford English Dictionary Wizarding Edition_  


It's nearly a month before the case is reopened.

The intervening time is hardly boring, to John's very great relief. Lestrade calls on an average of once a week, and in between there are the cases that Sherlock attracts using his website. Sherlock asks John along more and more frequently until all he has to do is throw his scarf round his neck and say "Coming?" with his hand on the doorknob, and John doesn't even bother to answer because of _course_ he's coming.

One morning in late February, John stumbles down the stairs in his usual pre-caffeine fog to find that most of the furniture in the sitting room has been, for want of a better term, melted. It is perhaps a sign of the moral degeneration that comes from living with Sherlock Holmes that John is merely bemused, rather than appalled. "May I ask what exactly you are doing?" he says, standing in the doorway. Sherlock is standing in the middle of the room holding his wand in one hand and a sort of white tube in his left. The tube is wired to a flattened box clipped to Sherlock's belt.

"I've written a new spell," Sherlock says. He flicks his wand at the red armchair, John's favorite, and rattles off something in Latin. The armchair melts into a puddle of red and brown, with various chunky bits sticking out at the top.

John boggles. It should not be possible for fabric and wood to melt that way; for a start, they'd burst into flames before they got hot enough. "What is that?" John asks as Sherlock drops the wand and removes the box from his belt to examine it.

"Infrared radiometer," Sherlock says, then crows, "19.4 degrees! Again!" He gestures at the chair's remains with the white tube and grins like a fiend. "Feel it," he says.

"No thank you," John says, eying the chair.

"It's absolutely safe. Feel!" Magic only seems mystical to Muggles who don't know any better; it still has to obey the laws of physics. The energy involved in melting that chair had to go somewhere, and the remains should be about a million degrees. They should be radiating enough heat to warm the entire flat. Yet the temperature in the room does seem perfectly comfortable.

John goes over and touches the chair lightly, then presses his hand against it. It's still room temperature. "That's impossible," John says.

"Very little is impossible with magic," Sherlock says archly. "At least, not if you're me."

John thinks about this for a second, and decides that the only way to deal with such an obvious display of egotism is to ignore it. He walks back through the kitchen and into the bathroom. And has to stifle a potentially embarrassing shriek when he sees the tub full of sprawling hairy stick-legs and round eyes and enormous mandibles. It takes one heartbeat to identify them, four (rather accelerated) heartbeats to realize they're not moving, and ten heartbeats to storm back into the sitting room.

"Sherlock," he says as calmly as he can manage. He is rather proud of the fact that he is barely shouting at all. "Our tub is full of spiders. Big ones." John spans his hands to demonstrate that when he says 'big' he in fact means 'gargantuan.'

"Infant acromantulas," Sherlock says, jotting notes into a lab book that he's balancing on his forearm. "Fascinating creatures. I'm testing the effects of-"

"Our tub, Sherlock. Is full of bloody enormous spiders," John says firmly. "Our. Tub. The one that I bathe in." This does not seem to be getting through to Sherlock at all. For a genius, he can be exceedingly stupid when he chooses to.

"There isn't another container big enough to hold them," Sherlock says.

John has coped with a great many weird things since he moved in with Sherlock. Letting himself be dragged away from a meal to poke dead bodies is fine. Being exposed to everything from poisonous fungi to sulfuric acid in their kitchen is also fine, now that he routinely wears rubber gloves when he does the washing up. Finding a pickle jar full of human eyes or fingers in the fridge when he's trying to make a sandwich, that's just his average Tuesday. But a bathtub full of giant arthropods is more than even John is prepared to deal with at 7 am on an empty stomach.

"Right," John says. "I am going to get dressed and go out for breakfast. And then to the shops. Sherlock." 

Sherlock actually glances up at John, for a wonder, and John makes the most of the eye contact. "I'm not worried about the furniture, I know you like sitting on it too, so you'll sort it out eventually. But I am deeply, deeply concerned about the spiders. They will be gone by the time I get back. Or else."

"Or else what?" Sherlock says curiously. He is desperately hard to threaten.

"Do not test me on this, Sherlock," John says viciously. "I mean it." He marches back up to his room to dress.

* * *

After a satisfying breakfast and a round of errands, John climbs the stairs to the flat and walks into the sitting room, thinking mostly about the need to get the frozen vegetables he'd bought into the freezer before they completely thaw.

There is a strange man sitting in the now-restored red armchair.

"Shit!" John says, and drops most of the shopping. His hands are ready to tense into fists, but the man is not moving and does not appear to be hostile. He's just sitting there in a ridiculously posh suit, with a ridiculously smug smile on his face, balancing an umbrella on his knees even though there's absolutely no sign of rain. "Sherlock!" John yells, but there's no response and John can see that the coat is missing from its customary hook.

"Not back yet, I'm afraid," says the man in the chair. He gestures with his umbrella. "Why don't you come in and have a seat, John?"

"I’m sorry, do I know you?" John asks. He gathers up the dropped shopping and carries it through to dump on the kitchen table.

The man’s voice rises to follow John. “This isn’t the polite reception I expected. Is this how you usually greet your clients?”

“Sherlock’s clients,” John corrects, coming back into the room. It’s true that clients sometimes turn up at the flat, but it’s far from ordinary for them to come inside while both John and Sherlock are out. Much less sit themselves down and invite John into his own damned sitting room. 

“My mistake,” the man says, tapping the handle of his umbrella with his forefinger. “I was given to understand you were working together these days. Colleagues, really.”

“I’m fairly certain that’s none of your business,” John says. He smiles tightly.

“Or just the man who pays half the rent?” suggests the stranger. 

"His friend," John says firmly. His hackles are starting to rise.

The man produces a full smile at that. "Really. Sherlock Holmes doesn't have _friends_."

"I think I'd know better than you," John rejoins. 

The smile ratchets up a notch. "No, I don't think you would," he says.

John briefly slips his phone out of his pocket to see if Sherlock has texted anything about where he’s gone or when he’ll be back. No, of course not. John wonders how long he’s going to be stuck here making small talk. When he glances back up, the man is still lightly drumming his fingers on the handle of the umbrella. "Why don’t you put that down," John suggests.

"I'm sorry?" The man raises an eyebrow but his hand stills- gripping the handle loosely, wrist turned toward John.

"Stop it," John says sharply. 

The man carefully removes his hand from the umbrella's handle and rests it on his knee. His other hand lifts the umbrella down and leans it upright against his leg. The tosser is still smiling. "Impressive," he says. "Most people don't realize it's there." 

John narrows his eyes at the man, whose smile doesn’t falter in the slightest. Then he hears the front door slamming, followed by Sherlock's footsteps taking the stairs two at a time. John turns toward the door without looking away from the man in the chair. Sherlock freezes in the doorway, a sour look flashing across his face before it fades to impassivity. "Oh," he says. "What are _you_ doing here?"

"Can't I check on you now and again?" the man asks lightly. 

"That's what your cameras and spies are for," Sherlock says, still from the doorway. "Don't flatter yourself that I don't notice your minions skulking about." He glances back at the stairs as if he's seriously considering going back out again.

"Don't be childish," the man says. "This paranoia is beneath you."

"As your compulsive meddling is beneath you," Sherlock counters. There's a brief pause so that the two can glare at each other, and John leaps on the opportunity.

"Ah," he says. "So you _do_ know each other, then."

"Of course we do," the man says.

"Unfortunately," Sherlock says in the same instant. The man gives him a look of disapproval. "Again, I ask you, what are you doing in my flat?"

"Our flat," John puts in. "He was in here already when I got here; I didn't let him in."

"Good," Sherlock says.

"Since you would rather forgo the social niceties, I am here to see if I might revive your interest in the murder of several chemical company employees last month," the man says. He lays his hand on a manila file folder John has just now noticed on the table beside him.

Sherlock looks positively gleeful. "You mean all the queen's horses and all the queen's men can't put the case together, so you've come to beg me to solve it for you," he says. "Well the answer is no, Mycroft. You let your hounds drive me off the scent and stand at the entryway to the last crime scene with a flaming sword. I'm not cleaning up your mess."

"You mix your metaphors terribly," the man- Mycroft- says, wrinkling his nose.

"Hold on," John says. "You mean the Aurors? This guy works for the Ministry of Magic?"

"He _is_ the Ministry," Sherlock says to John. "When he's not too busy being the Home Office, or the MOD, or MI6."

"Such an imagination," Mycroft says mildly. "I'm a _liaison_ , for goodness' sake."

"You're an interfering busybody," Sherlock says. "You have your thumb in so many pies it's a wonder you're not eighteen stone by now." He squints at Mycroft appraisingly. "Or are you?"

"Childish," Mycroft sniffs. He taps one finger on the file. "This is the _complete_ file on the chemical plant murders."

"So?" Sherlock glares.

"You cannot convince me that you haven't wondered what connects those four companies. What they have that's worth stealing." Mycroft smiles and removes his hand from the file. "You may even have thought that was the solution to the mystery. It's not. We already knew how they were linked."

John is sure he's not imagining the spark of interest in Sherlock's eyes; but the man's other features remain impassive, bored. "Go away, Mycroft, and take your nuisance of a case with you. I have far too much to do to waste time repairing your innumerable errors."

"Very well," Mycroft says smoothly, rising to his feet and picking up his umbrella. He leaves the folder where it is. "You know how to reach me. John, you may want to avoid that blonde nurse you've been chatting up at the clinic, she already has at least two lovers to my certain knowledge, and possibly as many as four." Mycroft strides down the stairs and John can hear the door close behind him.

"How does he know where I work?" John demands of Sherlock, who has gone to the front window and is now staring down at the street. "How does he know who I've been _talking_ to? Has he been spying on me?"

"Don't be stupid," Sherlock says. "Of course he's been spying on you. Probably read every file the government has on you, too; from your deployment history to your squib registration file."

"Who the hell is he, Sherlock?" John asks. "He walked into our flat, armed, like he owns it."

Sherlock turns back to the room and removes his coat, which he tosses over the back of the vacated armchair with a scowl. He flings himself down on the sofa. "He's my brother," Sherlock says snappishly. "And he's a git. Forget about him."

"Your brother," John says. Now that he thinks, he can see the resemblance in the way Mycroft had sneered. It was not entirely unlike Sherlock's own frequent expressions of disgust. John reevaluates the conversation in light of this fact. "In that case, wasn't it rather insufferably rude of you to insult him and throw him out of the flat?"

"Yes, but no less than he deserves," Sherlock declares. "And don't tell me you wouldn't have done the same. I've heard you speak to your sister on the phone."

John doesn't speak to his sister so much as he shouts at her, so there is not much John can say to counter this.

Sherlock crosses his arms over his chest and glares at the manila folder sitting on the table. "Damn him," he mutters.

"What?" John asks, who has finally started putting the groceries away.

"We never discovered the identity of the killer," Sherlock says. "There was not enough data to construct a viable hypothesis about the reason for the thefts, or any possible accomplices. Now, here is Mycroft, long after the trail has gone cold, to dangle the data in front of me like a tidbit."

John finishes putting the veg in the freezer and the milk in the fridge, and wanders back into the sitting room. Sherlock is sitting in the same position, still staring at the folder. "So you told him to shove off," John says. "What's the problem?"

"You can't imagine how an unsolved case _eats_ at me, John," Sherlock says in a low voice.

John can see the half-wild look of utter avarice in Sherlock's eyes as he stares at the folder, the fingers on his right hand twitching very slightly. He resembles nothing so much as an addict anticipating the next fix. "I'm starting to fancy I can," John says wryly.

With an obvious effort, Sherlock drags his gaze from the folder and picks up his latest lab book from the coffee table. John goes to the desk and opens up his laptop to check his e-mail.

Sherlock holds out for twenty-one minutes, by John's count, before he growls something obscene, slams the lab book back onto the coffee table, and snatches up the manila folder, thumbing quickly through it. John can't stop himself from grinning as he abandons his laptop and crosses the room to sit next to Sherlock on the sofa. John can see that every page inside the folder is indeed marked "top secret" and "eyes only." John wonders why Mycroft even bothers claiming to be just a liaison when he has access to things like this.

"Oh," Sherlock says breathlessly. "That bastard! Look at this." He passes a document over to John, who quickly skims through. His time dealing with both hospital administrators and written Army orders has made him passingly fluent in beaurocratese, so it only takes him a few minutes to parse what he's reading.

"These are specs," John says. "Some kind of technical project for the MOD. All four companies targeted were working on it."

Sherlock continues to flip through the folder, pausing now and then to speed-read a particular document. "Exactly."

John lingers over the word _aerolization_. "Is this a-" He licks his suddenly-dry lips. "Is this a chemical weapon project?"

"No, no," Sherlock says. "Merely laying the technical groundwork for one. Apparently it began in 2002."

"Jesus Christ," John says. He supposes he shouldn't be surprised; British companies supposedly exported precursor chemicals to Iraq in the 80s, and nobody seemed to respect the Geneva Protocols much these days, if his own experiences were any indication. But the idea of his government moving in the direction of chemical weapons is deeply abhorrent.

"Over now," Sherlock notes. "Axed in 2005. Mycroft's name isn't mentioned, but the termination order has his sticky fingerprints all over it."

"Good on him," John says, and his estimation of the man rises sharply.

Sherlock drops the folder to the table and steeples his fingers. "Sometimes the back door is the easiest way in. Security guard, secretaries, janitor. All low-level employees, yet trusted with routine access to the facilities where important, secret documents were stored. I always suspected that the killer was after trade secrets of some kind, but now we know what exactly he wanted."

"Records of this project," John says. "Blackmail?"

"No," Sherlock says. "He wanted the plans for their own sake. He wanted to put this research into practice. Which, along with his obvious stupidity, precludes a solo effort. There are co-conspirators involved in this."

John sets the document he is still holding gently back on top of the stack in the folder on the table. "Are you going to help find them?"

"Of course," Sherlock says. "The game is back on, John. I shall have to find another way to express my annoyance with Mycroft." Sherlock's eyes are shining, and John smiles to himself. Mycroft was obviously a smart man- at this point you wouldn't be able to drive Sherlock off the case with a cudgel.

"They wouldn't have stolen plans to aerolize a chemical if they didn't have something in mind to aerolize. The sloppiness of the murders suggests either a need for haste or an inability to be subtle. Either way, they will have put whatever it is into production," Sherlock says thoughtfully.

"So, careless idiots or incompetent morons?" John asks, cracking a smile.

"Essentially." Sherlock steeples his fingers. "Their lab will be small, housed in someone's basement or in a vacant building."

John glances at the kitchen, which contains Sherlock's laboratory equipment and several experiments in progress. "That sounds dangerous."

Sherlock rolls his eyes. "I am manifestly not an idiot. Nor am I careless or incompetent."

John considers debating him on the issue of carelessness. "Accident-prone?" he suggests as a compromise.

"John. There are _always_ accidents in a laboratory. Most of them are minor, but the severity of any mistake increases based on the danger presented by the material involved."

"Ah," John says, comprehending. "And based on the carelessness of the chemist, I suppose?"

"Precisely." Sherlock grabs his wand off the top of the bookcase and goes for his coat. "Come on, we need to talk to Lestrade."

* * *

"Why exactly should I help you?" Lestrade asks.

John tries not to laugh at the poleaxed expression on Sherlock's face.

"It’s for a case,” Sherlock says tartly after a moment: his go-to excuse for nearly everything.

"I’m sure" Lestrade says. “And which case would that be?”

"Official Secrets Act," Sherlock says promptly.

John coughs to cover a laugh, and Lestrade rolls his eyes. "Sherlock, you keep refusing _my_ cases because they're too boring. And then you swan in and demand a list of every suspicious injury and death in the past month? We don’t keep lists like that, someone would have to make one. Someone in my division. Who, unlike you, is actually working on murder investigations at the moment." Lestrade leans back in his chair. "So. Give me a reason, a good reason, why I should do you this favor?"

“Because you’re a dedicated public servant and this is a case of national import,” Sherlock rejoins.

“And I’ll just have to take that on faith, won’t I, since you won’t tell me anything about it?” Lestrade asks, raising an eyebrow.

“I don’t have time for this,” Sherlock says, and puts his hand on the edge of the door.

"Come on, Sherlock," Lestrade says. "Is it really that hard?"

"One cold case," Sherlock says promptly. "My time is valuable, and I'm in the middle of something."

“My people’s time is valuable too,” Lestrade says. "Five."

"Two," Sherlock snaps, crossing his arms.

"You're the one asking the favor, you don't get to bargain,” Lestrade says. “Five."

Sherlock glares at Lestrade for a long moment before he pushes out, " _Fine_ " past gritted teeth.

Lestrade picks up his phone and punches three digits. "Donovan, could you come in here for a second."

Donovan's scowl matches Sherlock's when she opens the door and sees him standing there. "Yes, sir?" she says to Lestrade.

"I have a project for you," Lestrade says. "I need you to go through our database and make some calls. We need a list of suspicious injuries and deaths in the past thirty days."

"Is this for a case, sir?" John knows the look on Donovan's face well: it's the look of someone who has just realized her productive afternoon in the office has gone to shit.

"Not as such," Lestrade says.

"Oh," Donovan says scathingly, glaring at Sherlock. "So I'm going to be spending the day with the Freak, am I?"

Sherlock's eyes snap and he opens his mouth, but Lestrade hurries to cut him off. "No, Sherlock's going to go through a few case files for me. He'll tell you what he needs, and you can give him the list after he's done here."

"Why don't I go with Sergeant Donovan?" John asks. "I know what Sherlock's looking for, so we can narrow it down a bit. I don't fancy sitting here watching you two read files, at any rate."

Sherlock nods grudgingly. "All right," he says.

Lestrade nods as well, so John gets up and follows Donovan back out to her desk. "So what are we looking for?" she asks resignedly, picking up a pen and dragging a pad of paper closer.

"A covert chemical lab," John says. It's probably fine to admit that much. "Sherlock figures a setup like that will have its share of accidents, so we want to look at injuries and deaths that could be traceable to laboratory errors."

Donovan taps her pen against her upper lip. "That's casting a pretty wide net," she says. "We're looking at explosions, first, I suppose."

John nods. "Yeah. And serious burns. Poisonings. You can get cut in a lab too, of course, quite easily, but if we look at lacerations we'll be here forever."

"All right," Donovan says, making notes on her pad. "Come here and I'll show you how to search in the Met's citywide death database. While you do that, I'll get on the phone with some other divisions and look into the injuries angle."

Looking through the database and noting down cases that fit the criteria is unsurprisingly boring. About mid-afternoon, John takes a quick trip to a nearby deli just to have an excuse to stop doing it for a while. Delivering Lestrade a ham sandwich on rye is also a good excuse to make sure he and Sherlock haven't quietly murdered each other.

"Really, Lestrade, you're given an entire day of my time, and these are the hardest cases you can find to present me with?" Sherlock is gloating so hard, you'd almost think the whole thing was his idea.

"Mock me if you like, but that's still four more murderers we'll have off the streets," Lestrade says.

"You'd best make the most of your last request, detective inspector," Sherlock says. "For your sake and for mine. I'm in danger of being bored to death."

Lestrade's cheek twitches, and then he smiles. "Bored, are you?"

"Dreadfully," Sherlock says. "Show me something worthy of my attention. Or are all the murderers in London really so artless?"

Lestrade thumbs slowly down a stack of files piled on top of his printer and pulls one out, balancing the rest of the pile with his free hand. He thumps the two-inch thick file down in front of Sherlock. "See how you like this," he says. "Twenty-three year old male, identity unknown. His body was dumped outside St. Thomas' A&E about a month ago. No wounds, no physical trauma."

Sherlock opens the file and starts flipping through the reports. "What did toxicology show?"

"Nothing we know how to identify," Lestrade says. "There's definitely something there, some kind of neurotoxin. The pathologist found that death was caused by paralysis of the lungs and heart."

“Did they check for any specific toxins?” John asks. “Botulism, tetanus-?”

Lestrade shakes his head. “Yeah, but the pathologist said nothing quite fit.” 

Sherlock finds the pathologist's report and passes it across to John, who reads silently for several minutes. Sherlock keeps flipping pages. "Oh, they found hemotoxins too," John says. "So it’s a snakebite, probably. Unusual in London, but what makes you think it's murder?"

"There's no bite," Lestrade says. He leans forward and pulls a packet out photos out of the back of the file, then unwraps them and hands them over to Sherlock. "But they did find needle marks inside his left elbow."

John flips back a page in the report. "There was significant necrosis in the left hand, this says. That could be consistent with a venal injection site."

"Possibly," Sherlock acknowledges. He's shuffling through the photos now. He flicks two photos onto the desk in front of him and keeps shuffling. "Flame tattoo on the left shoulder blade."

"The report says he would have died in minutes. His body was still warm when they carried him into the A&E," John says. "What snake's venom kills that fast?"

"Most of them," Sherlock says. "It all depends on the dosage. Ah!" He stops shuffling and spreads out several of the photos on the desk. John bends to look. They show close-up shots of the needle marks in the young man's arm. Sherlock flips a few more photos, and then his face freezes. "Oh, you idiots," he says breathlessly. "You colossal- why didn't you call me in on this sooner, Lestrade?" Sherlock flings the stack of photos down and slaps one hand flat on Lestrade's desk with a ringing sound. 

"Not boring, is it?" Lestrade asks, and grins a bit. "It wasn't my case till the day before yesterday, when the original DI gave up and asked for my input. This is Toby Gregson's case, and you know he would rather cut off his own arms than ask you for help."

Sherlock scowls. "A month! A bloody month and I could have been- of all the incompetent- you have no idea how much time your colleague's petty posturing has wasted!" He's working himself into a proper passion.

"I assume you've found something, then?" John says.

Sherlock picks up the photo on top of the stack and shoves it at John, who takes it carefully in hand. It's a close-up of the damage to the victim's hand: the tip of the left index finger is blackened, and the blood vessels surrounding it are black as well, their lines jagging up the hand and onto the arm as if someone has traced the veins with ink. John frowns at it; something about the image almost jogs his memory.

"Well?" Sherlock says impatiently. "The speed of action? Necrosis at the wound site? The characteristic darkening of the veins?" John shakes his head slowly. "Runs up your arm like a black, black road-"

The rest of the pernicious childhood rhyme echoes in John's head- _Laid by a chicken and hatched by a toad_ \- and he suddenly _gets_ it. "Oh!" he says. "Oh shit."

* * *

Of course they have to wait until they're safely out of the Met and ensconced in a black taxi with the partition shut before either of them can actually voice the problem.

"How the _hell_ is someone keeping a basilisk in London?" John demands.

"Not necessarily the beast itself, just the venom," Sherlock points out.

"Either way," John says. "Aren't they banned? Nobody's kept one in- god, hundreds of years."

"2004," Sherlock says. He's perched on the cab's fold-down seat facing John, pressing one thumb against his lower lip and looking entirely too unconcerned for John's tastes.

"Excuse me?"

"The most recent sighting was in 2004. A basilisk was found living under Hogwarts," Sherlock says. Sherlock gives him an arch look.

"Afghanistan," John says, rolling his eyes. "Anyway, what's in Hillcourt Circle?" Sherlock had barked the address as they got in; it's not one John recognizes.

"The former clubhouse for a wizarding street gang," Sherlock says.

"There are wizarding gangs?" John asks. Sherlock gives him a look, like _of course_ there are wizarding gangs, and why doesn't John know about them? John wonders how Sherlock knows about them, they can't turn up in Muggle crime all that often. "How silly of me. So what do they do then, apparate in and out of bank vaults?"

“Nothing so high-minded," Sherlock says. "They're wizarding supremacists. Neo-Voldians, I think is the term now, but they existed prior to and contemporaneously with the wizarding wars. Angry, antisocial youth are a traditional recruiting ground for revolutionaries and terrorists." John blinks a bit at hearing Voldemort described as a revolutionary.

"And you're guessing our poisoning victim was one of them? Because...raising a basilisk is dark magic, and that suggests a connection to these street gangs?" John still isn't quite following the train of logic.

Sherlock rolls his eyes. "I don't _guess_ , John. No, there are nine separate clues on the body indicating that our victim was the member of a gang, it's abundantly clear from his wand callous that he was a wizard, and of course the nature of the gang is suggested by a marker so prominent I would have expected even you to spot it." John crosses his arms over his chest, feeling a bit stung by that. "His tattoo, John! Flame is not an uncommon subject of the art, but in this context it is highly suggestive. Flame is the symbol of the ancient holiday of Walpurgis, traditionally celebrated with bonfires as a night when the line between life and death grew thin and witches met in their covens. Walpurgis formed the inspiration for the Knights of Walpurgis, who used as their symbol a stylized flame design strikingly similar to this one, and who in the 1970s and 80s became closely identified with, and ultimately subsumed by-"

"The Death Eaters," John finishes.

"Exactly," Sherlock says with relish. "So the flame tattoo identifies its wizarding bearer with neo-Voldian thought.”

***

The "former clubhouse" is in a council estate that's seen better days. When Sherlock knocks at the door of a particular flat, a whippet-thin man in his early 20s with greasy hair down to his shoulders answers.

"Ah, Canton," Sherlock says, and deftly puts his foot in the door to prevent it from slamming closed. "A word, if you would."

"What's wrong with you?" Canton demands in a hiss, giving a quick glance up and down the hall. "There's other wizards what live in this building! You trying to get me hexed?"

"Why don't we talk inside, then?" John suggests. The man turns his attention to John, a momentary distraction that allows Sherlock all the time he needs to push the door open and bull his way in. John slides in behind. The flat inside is a dim cavern of battered Ikea furniture, piles of dirty washing, and ancient foodstuffs best not contemplated too closely. A thin haze of tobacco smoke fogs the room.

Canton quickly shuts the door behind them. "Five minutes," he declares, crossing his arms.

"I need to know which of the gangs is using the Walpurgis flame as their symbol," Sherlock says.

"I don't run with them bastards no more," Canton says insistently, his voice low. His wide, darting eyes betray a nervousness at odds with his confrontational posture. "I'm off the dope, I don't even drink, and I ain't with them bastards."

"I know, Canton, and that's why I need your help," Sherlock says. His voice is gently persuasive, asking rather than demanding. It's shockingly humble of him, and not for the first time John wonders at the history between Sherlock and his ragtag informants. "No one else in the Fists would tell me anything, would they?"

"Not to a blood traitor, no," Canton says. He fishes a packet of cigarettes out of his pocket; his hands are shaking so badly that he can barely keep the match steady enough to light it.

"I just need the name of the group, where to find them," Sherlock says. "They're up to something, Canton. One of them is dead and no one claimed his body."

Canton nods, jerkily. "They're scared," he says.

"Are they," Sherlock says evenly. He perches on the edge of a battered sofa that looks like its about to rise up and envelop him like an amoeba. John elects to stay standing.

"Well they're working with someone outside, ain't they? And not no wannabes, neither. _Proper_ dark wizards. Some as says maybe they even used to be- _you_ know."

"Yes," Sherlock says. "I do know. Just tell me who and where, Canton, I won't ask anything more."

Canton sucks desperately at his cigarette. "I dunno Mr. Holmes. They came to our mum's. I did six months in Wandsworth and on the street people still say I'm a rat, you know? I don't have no wizard education so I applied for a Muggle job, and they came to our mum's and said I'm a blood traitor. If anyone finds out you was here-"

"I'll take care of it," Sherlock says. "Blood traitors have to look out for each other. Tell me who. And where." His voice is steely sharp. Canton flinches and stubs his cigarette out against the arm of his chair.

"All right," the man says. "All right. Masingberd's his name. Trent Masingberd, fancies himself a real wizard, him and those shites. Got the tattoo right on his face, to show he don't care about nobody and he's in it for life, right? There's a place out in Lambeth where they was doing something. I don't know what. I don't wanna know."

"Excellent," Sherlock says, nearly springing to his feet but still not losing his air of calm. Now it's an eager calm, and John can sense the frenetic energy waiting to burst out. The game is on.

John waits till they're in another cab. "Blood traitors have to look out for each other? Really?"

"Well look at the company I keep. I'd say that's pretty indicative, wouldn't you?" Sherlock says, deadpan.

"Oi, thanks a lot," John says, but he grins at the window and watches Sherlock's reflection grinning back.

* * *

It turns out that locating a wizarding supremacist isn't as easy as checking the phone listings, even if he is living in Muggle London. Thanks to Sherlock, John becomes intimately acquainted with several dive bars before finding the one that Masingberd's boys evidently consider their local. Flame tattoos are on full display amongst the contingent of thugs who crowd a corner table and glare menacingly at anyone they see glancing in their direction.

Masingberd holds court with a pint and an endless series of cigarettes, finally dispatching one of his lads on some errand with much gesturing. It's that one that Sherlock elects to follow, and as usual he's on the right track: the minion leads them to a warehouse in Lambeth. Sherlock is almost vibrating with impatience the entire time the man is inside, and practically runs to the door when he's safely gone. The door has a locking charm, and Sherlock happily sets to work opening it with a settle of Muggle tools. “Wizards,” he says scathingly when the door clicks open with the charm undisturbed.

Inside is a perfectly ordinary warehouse, sprawling and full of pallet after pallet of cardboard boxes. The interesting bit is the small office near the entrance. “This is where he just was,” Sherlock says, touching the light bulb on the one of the two desks. “Still warm.”

“So what's in here that matters to that lot?” John asks, flicking the switch to turn the light back on. He has a look around the office, but there's not much to it. Boxes stacked in the corner, paperwork scattered on the desk- looks like cargo manifests and customs forms for batches of rivets. Hardly the sort of trade street gangs got involved in.

“Ah!” breathes Sherlock. John turns to see Sherlock crouched half under the other desk, brandishing what looks like a hip flask.

“Liquor smuggling?” John guesses.

“I doubt it,” Sherlock says. He stands up, slamming something under the desk shut with one hand and weighing the flask in the palm of the other. “Too heavy, but the outer shell is ordinary. Must be the lining.” He twists the top off the flask and sniffs delicately at the rim.

“Please tell me you're not going to taste it next,” John says.

“Hardly.” Sherlock tips the flask just enough to allow a few drops to fall. The liquid hits the floor, sizzling like a drop of water on a hot pan, and rapidly produces a black-edged circle in the cement, the size of a five-pence piece.

“Is that the _venom_?” John demands. “Jesus, I can't believe you're splashing it around like that, are you insane?”

Sherlock recaps the flask. “Well I needed to confirm somehow. There isn't a litmus test for basilisk venom, although I probably could develop some kind of-”

“Are they producing it here?” John asks, tensing. “Please don't tell me there's a bloody basilisk in this warehouse.”

“Relax, John,” Sherlock says, pocketing the flask and coming over to John to sift the papers he's already looked at. “It's probably synthetically produced-”

“You can do that?” _Why would you want to_ , might be a better question.

“Yes, of course, don't interrupt,” Sherlock says. “But not produced here, obviously. This seems to be a sample of some kind.”

John moves over to the stack of boxes and carefully slits open the tape on one with his pocket knife. “Huh,” he says. “Bullets.”

Sherlock walks over, glancing between a paper in his hand and the label on the side of the box. “A bit less legal than rivets,” he notes. “Open another.” The next box is full of closely packed boxes of ammunition as well. Sherlock finds another manifest and they hunt down and open another couple of the boxes listed, with the same result. He charms the cut seals of the boxes back together and they return to the office, where Sherlock ruffles his hair and glares at the paperwork on the desk as if trying to make sense of it. “It can't just be arms trafficking,” he mutters. “What would neo-Voldians want with bullets? _What_?”

John leans on the corner of the other desk and pries open one of the packs of ammo. "I thought wizarding supremacists were deadset against Muggle weapons in any case,” he remarks. “And if they're hoping to encourage all the Muggles to kill each other, it seems there are enough bullets being produced without-"

"John, stop!" Sherlock says sharply. His voice is so urgent and alarmed that John instantly freezes, his free hand stopped above the box of bullets in the act of reaching inside. Sherlock walks over to John and takes the box out of his hands. He removes a bullet from the box, handling it awkwardly with his thick leather gloves, and holds it up so John can examine it. "What kind of bullets are these?" Sherlock's expression is intent, but his eyes are glowing with the mania of a puzzle solved.

"Cartridge, technically- 5.56 millimeter rifle," John says almost automatically, then frowns as he notices the characteristic dip in the head of the ammunition. "Looks almost like a hollow point, actually, but that doesn't make sense. Assault rifles are military application only, and the military doesn't use-"

Sherlock drops the bullet back in the box, then drags a familiar flask out of his pocket and shows it to John. He raises an eyebrow and waits, as if expecting John to connect the rest of the pieces. This is one area where John is more than able to keep up, although the inevitable conclusion is so horrifying that it takes him almost a minute longer than it should to reach it.

"Those-" John swallows. "Those bullets have basilisk venom in them."

"Yes," Sherlock says gravely. "In the hollow inside the jacket. The venom is corrosive, but of course lead is highly resistant."

John unconsciously scrubs his bare hands against his hips. The poison is inside the bullets, naturally, but there can always be leaks, and John is _deeply_ glad that Sherlock stopped him from touching the things. "Christ," he says hoarsely. "Do you realize what this means? Every bullet wound, one hundred percent fatal, every time." His mind flits across dozens of faces, dozens of patients whose lives he'd saved in Afghanistan. If they'd been hit with bullets like these, nothing he could do would have made any difference to them. They'd all be dead. _John_ would be dead. His stomach lurches. The idea is obscene, almost incomprehensibly horrific.

"Yes." Sherlock slips the bottle of venom back into his pocket. "It's an obvious application, really, if one has access to mass manufacturing capability," he says thoughtfully. "These weren't machined locally, someone must be having them made elsewhere and then shipping them to London. Should have occurred to me sooner, really."

"It wouldn't occur to any sane person," John says harshly. "It's been illegal to magically modify ballistic weapons and ammunition since the Vienna Accord of 1919. Every European nation's Ministry of Magic signed it."

"Really?" Sherlock says, jiggling the box and studying the bullets inside. "Sensible of them."

"You didn't know that?" John asks, momentarily distracted by yet another startling gap in Sherlock's basic knowledge. "Honestly, Sherlock, did you even _go_ to school? I haven't had a History of Magic lesson in my life and I still know that."

Sherlock looks up and shrugs. "Not relevant," he says. "If I ever learned about it, I probably deleted it." This is not the first time in their association that Sherlock has off-handedly mentioned “deleting” facts from his “hard drive.” It's a metaphor that John finds irritating, given that it's always being employed as an excuse for not knowing things like when the water bill is due, which inevitably become John's responsibility by default.

"Well it's relevant now," John says fractiously.

Sherlock shrugs and pockets the box of bullets and the flask. "Perhaps," he allows.

"Go on," Sherlock says in the cab home, after they've safely resealed the boxes and shut up the building behind them. "Tell me."

"Tell you what?" John asks, startled from his reflection on Sherlock's coat and its pocket full of tiny war crimes.

"What led to the Vienna Accord. Correct my ignorance," Sherlock says. His voice is sarcastic, but he's watching John very intently.

"During World War One, the Austrian Ministry of Magic broke the International Statute on Wizarding Secrecy in a big way," John rattles off. He can practically recite the story from memory. "They put Healers in their field hospitals and made some important strategic points unplottable. But the biggest thing they did was get wizards involved in arms production. They enchanted bullets to be invisible, to explode.” John swallowed. There’d been pictures- the carnage had rivaled that created by modern IEDs. “They hid it for a while, but it all started to come out in mid-1918. That was the real reason the Allies forced the breakup of the Austria-Hungarian Empire. And good thing, too."

"You read about this," Sherlock says.

"Yeah," John admits. "My mum got me some books when I was studying the war in school." Even then, she had been needling Muggles for an ignorance that was deliberately induced by the wizarding world; John had been struck by the unfairness of it but he didn't criticize his mum, then or ever. "Christ, what the hell do wizards want with magically-modified bullets, Sherlock?"

"What do wizards want with a mechanism for aerolizing liquid chemicals?" Sherlock answers back, and John goes cold.

"You think this is connected. To those chemical plant murders," John says slowly.

"There are no coincidences, John," Sherlock says mildly. "Merely convergent streams of data."

John thinks again about those books he read years ago, with their alarming pictures. "The Austrians," he says, and Sherlock looks at him. "They also had mortars that spewed Garroting Gas. Do you think some nutter wants to aerolize potions, maybe?"

"Not a nutter," Sherlock murmured.

"Sorry?" John says.

"Designing something dangerous doesn't make one crazy, John," Sherlock corrects. "It's brilliantly inventive, really. Can you conceive of how lucrative such a development could be in the modern arms market? Governments and insurgents alike, clamoring for the latest innovation in controlled murder. Assassins and common murderers. Imagine the merry hell an invisible bullet would play with police forensics." Sherlock's eyes are lit from within, sparking as they only do when he is both fascinated and engaged. In this specific context, it makes John feel rather ill.

"Stop it," John says. "You're talking about _war crimes_ , Sherlock. Unbelievably horrific acts of- these things are banned for a reason!"

"Fear," Sherlock acknowledges. "Perfectly rational fear, I admit. But it doesn't stop the theory from being brilliant or the design from being elegant." He puts his hand in his pocket and John imagines him fondling the box of bullets.

***

John doesn't sleep very well that night.

He gives it up at four am and comes down to the kitchen where Sherlock is opening up one of the seized bullets, his hands carefully gloved. The others are in their plastic bag on the counter. John sits across the table from Sherlock and stares at the bag.

"Stop thinking so loudly," Sherlock says, picking up a pipette.

"Doesn't it bother you at all?" John asks.

"Why should it?" Sherlock carefully siphoned the liquid out of the bullet and dripped it into a glass test tube.

For a moment John is staggered by this; then he wonders why, it's not like Sherlock even knew why the bullets were illegal in the first place. "Because it's horrible. Unnatural. Because someone is out there making these, which means someone looked at war, at the shit people do to each other, and wondered, 'How could I possibly make this worse?' And that's- I can't even conceive of wanting to do that. It's evil."

Sherlock is frowning now. "Invention isn't evil, John. It's amoral. A very great deal that is valuable has been discovered from the pursuit of pure science, not to mention attempts to develop technologies useful in waging war."

"What's the difference, if the result is the same?" John argues. "At the end of the day you've still helped destroy people. You've- unleashed something dangerous." Sherlock raises and drops one shoulder, an unconvinced shrug.

Suddenly unable to keep still, John stands up and begins to fill the kettle. "I think we should go to the Ministry about this."

"Why?" Sherlock asks, sounding surprised.

"Because tampering with Muggle weapons this way is deeply wrong and deeply illegal!" John successfully resists the urge to add _obviously_. "Because we can't involve Lestrade, and this is fucking dark magic, for God's sake."

Sherlock chuckles. "Let me save you some time, John. You'll go to the Aurors because that's what your sort do. You'll explain the case and they'll tell you sorry, no one's been shouting Unforgivable Curses so clearly Dark wizards aren't involved, have a nice day. You'll try the LEP next, with much the same result. Unless of course my name comes up, in which case whatever poor bastard they delegate to talk to you will have an apoplectic fit. Net gain, zero."

"And your solution is what, exactly?"

"Track the importation of the bullets, find out where they're coming into London. Follow the chain to the person who controls the project, who will be the same person who directed the chemical plant murders or will point us in that person's direction."

"Alone. Without backup," John says flatly.

"I have you for backup."

John tries not to feel warmed by that, he really does. "You are insane. And you can’t use flattery to manipulate me, you know.”

Sherlock waves a temporarily free hand at John. "Fine, waste your time talking to the Ministry if you like. Don't let me stop you." He picks up a fresh pipette and uses it to steadily drip some clear fluid into the tube of venom. "When you realize that I'm right and you've wasted your entire morning, I will be here. Doing _useful_ work."

***

John hates going to the Ministry for several reasons, but he encounters one of the more irritating ones the minute he gets in. "Gotta check your wand, sir," the gawky young wizard at the security booth informs him.

"I don't have one," John says, but the security wizard's eyes narrow.

"What, did you forget it or something?" he scoffs. "I need to weigh it, sir. Rules. More than my job's worth."

"I didn't forget it, I don't have one," John says. It's clear the wizard still doesn't believe him, and a line is forming, so he sighs and adds, "Yes, all right, I'm a _squib_."

The flick of expressions across the boy's face- shock, revulsion, pity- is tediously familiar. "Got your card, sir?"

"I don't have to carry it any more," John says instantly. One of Kingsley Shacklebolt's more recent reforms was a repeal of the Squib Registration Act. And yet...

"No sir, but how am I supposed to know you don't have a wand unless you can prove it?" _If the Minister of Magic says you can take my word for it, who are you to second guess_ , John wants to say; but he knows the importance of picking one's battles, and having a blazing row with this young idiot is not going to accomplish anything.

John pulls his card out of his wallet and slaps it down on the counter. The wizard inspects it carefully. "Right, sir. If I can just-" He waves his Probity Probe vaguely and John nods and allows himself to be scanned for contraband magical items. Finally he's allowed to pass the booth and take the lift.

He decides to take Sherlock's advice about one thing; he's _not_ going to go to the Aurors and risk running into the ever-suspicious Perryvall, who apparently has something in for Sherlock. Instead he heads for the LEP Office and tells the receptionist that he has a crime to report, and after a lengthy wait he is let in to see a middle-aged wizard named Hayward Skipwith.

Skipwith cycles through politeness to skepticism, and is all the way down to condescension by the time the conversation is over. John of course hasn't brought the bullets as evidence- they'd set off the Probe at security, and how the hell was he going to explain having something that illegal?- and Sherlock's scribbled notes and proofs just seem to baffle the wizard. By the time Skipwith patronizingly advises him to leave investigations of illegal weapons possession to the Muggle police and keep his pet theories to himself, John is thoroughly fed up with the whole thing.

"You should stay well out of it too,” Skipwith says with all the stern condescension of a seasoned officer talking to a civilian. “I'm sure the Muggle police are competent to handle things without the aid of a medical doctor and a- what does your flatmate do?"

"He's a consulting detective," John says, trying not to grind his molars while he's actually talking. The wizard's jaw drops, and John belatedly remembers why he was trying to avoid mentioning Sherlock.

"Merlin's beard!" Skipwith says with barely-concealed horror. "Your flatmate is _Sherlock Holmes_?" He opens his mouth again, as if he wants to say something else, then snaps it closed.

"Well, thanks for your time then," John says.

"Not at all," Skipwith says briskly. He does not stand. "Send an owl if something else comes up, please. You can see yourself out?"

John staggers slightly when he stands to go. He tries to tell himself that the "bad" leg has just fallen asleep, but he's still limping when he gets home.

"Ministry went as expected, then," Sherlock says, not looking up from the notes spread in front of him on the coffee table. "I did warn you."

"Yes, thanks for that," John says irritably. "Have you seen my cane?"

"You don't need a cane, your injury is psychosomatic," Sherlock says.

"Tell that to my bloody leg!" John snarls. He grips the banister very tightly on his way up the stairs to his room.

* * *

Mycroft is waiting in the flat when John gets home from work the next day.

"Hello again," John says, hanging up his jacket. "Am I to expect you to turn up in our flat every time I pop out, then?"

"You will be pleased to hear that your efforts at the Ministry have resulted in an LEP task force being formed," Mycroft says.

John looks sharply at his face, which betrays nothing. "Really? That's funny, because I had the distinct impression, while I was down there, that I was being blown off."

"How did you expect the authorities to react to a squib who claims to be investigating magical crime?" Mycroft says blandly. John feels a sharp stab of concentrated loathing, and pushes it firmly away. "Fortunately a few judicious words persuaded our intrepid law enforcement officials that the matter deserved a closer look."

"Your words, I suppose," John says, resisting the urge to massage his thigh. "You couldn’t have acted earlier, and saved me a trip?"

"Not before I had ascertained the extent of Sherlock's...involvement," Mycroft says. 

John's eyes dart from the door to the side table where he last saw Sherlock's wand, and he wonders when he can expect his flatmate back. This is his bloody brother, let Sherlock deal with the bastard.

"Ah," Mycroft says quietly. "You haven't told Sherlock about your little visit to the Ministry, then."

"Of course I have, we discussed it before I even went." John blinks and rewinds the conversation. "You didn't come here to gloat, you came here because you thought I- what? Sneaked to the Ministry on your brother?" He can feel his cheeks and forehead staining red with suppressed anger. "What the hell do you think I am? I told you I'm his _friend_."

"And I told you," Mycroft says coldly, "that Sherlock Holmes doesn't have friends."

"Just a controlling older brother, is that it?" John demands.

"Protective, Doctor Watson, is the word you want." Mycroft is stroking the handle of his umbrella, but he seems to have learned from last time: it's leaning lightly against his thigh, and his other hand is on the armrest. "It is easy to set a wheel in motion; much harder to slow it to a stop. And the Ministry contains some very large wheels indeed. Do you take my meaning?"

John thinks about the way the Aurors went after Sherlock during that first case, how clear they made it that they didn't like him and found his involvement suspicious. He feels a bit ill, wondering if maybe he has brought the wrong sort of attention to Sherlock; but if that was the case, why had Sherlock not tried to stop him from going to the Ministry? Nonetheless, Mycroft seems to be warning him that his reach in the Ministry is not all-inclusive, whatever Sherlock says. "Yes," he says.

Mycroft abruptly stands. "I have come for the bullets," he says. "I presume my brother has finished his...experiments." He says the word 'experiments' so distastefully that John might laugh, if he wasn't so angry.

"If he hasn't, that's too bad," John says. He locates the plastic bag of bullets among the detritus on the kitchen table and thrusts it at Mycroft. "Here." It will be a relief to have them out of the flat. Their presence makes him deeply uncomfortable in a way that human body parts and mold cultures never could, and if Mycroft wanted to dispose of the things, he's welcome to them.

"I appreciate your cooperation, Doctor," Mycroft says.

John bares his teeth. "Don't," he snaps. "The only reason I'm giving you these is because I want them, and you, out of my home as quickly as possible."

* * *

One thing John has learned about detective work is that even Sherlock's procedure-free variation involves a heavy amount of waiting for things to happen. Of all the various kinds of waiting- waiting for rescue, waiting for another murder to happen, waiting for a source to call, waiting for Sherlock to tell him what the hell is going on- John's favorite is probably the stakeout, waiting for criminals to show up while you watch. It has all the regular tedium that waiting implies, but it also skates on the edge of action at all times, a sort of foreplay that the adrenaline junkie in John craves. So when Sherlock says "Stakeout, Masingberd's warehouse, come on," John just says, "I'll get my jacket."

It's already well after dark when they get there. No convenient trattorias or coffee shops decorate this industrial district, so they're stuck concealing themselves in an alley across the way, under a darkened street lamp. Whatever Mycroft said about the LEP taking an interest, there certainly seems to be no law enforcement presence here tonight. John and Sherlock trade a set of magically-enhanced binoculars back and forth, and John risks Sherlock's wrath by turning his gaze away from the loading dock. The entire street is deadly quiet, with no sign of wizards, dark or otherwise.

Sherlock makes an impatient noise and takes the binoculars away. "What exactly are you expecting to happen tonight?" John murmurs.

"Movement," Sherlock says in a similar undertone, his voice so deep as to render his words almost inaudible. "The bullets have been there too long, and there's talk on the street that Masingberd and his associates are growing restless. They'll have to move the bullets soon."

"Are they really smart enough to realize how at risk they are?" John asks.

"It's not a question of intelligence," Sherlock says. "They're idiots, but they're cunning idiots. Masingberd will move the bullets soon. Tonight, or at the latest tomorrow night." John stifles a groan at the prospect of another cold, damp night spent crouching in this alley. If they have to come back, he's bringing tea at least.

It's John's shift with the binoculars again when he spies movement. "Sherlock-"

"I see it," Sherlock says tersely. Car doors slam unbearably loudly in the quiet street, and dark figures hurry past the loading dock and into a side door. John focuses the binoculars and sees a glimpse of a face bearing the distinctive tattoo.

"It's him," John mutters. "I'm going to call in the Ministry- they can send us some backup, at least." Sherlock wrestles the binoculars away easily while John is fumbling one-handed for his phone, and examines the cars while John makes a quiet call to Mycroft. Sherlock's brother may not be John's favorite Ministry employee, but he's the only one whose cell phone number John has access too. He turns away from the street to hide the light emanating from his phone, and when he turns back Sherlock is simply gone.

He opens his mouth to call out, then spots him darting across the street. 

Sherlock is tugging at the rolling door of the loading dock when John reaches him. "Sherlock," John says sharply. "Come on. The plan was to wait for the LEP to get here, remember? The trained law enforcement agency?"

"That was your plan," Sherlock says. "Not mine." He manages to haul the door up to about chest high, and looks back at John. "Coming? Or you're welcome to wait if you like."

John grits his teeth, but his cooperation is a foregone conclusion and Sherlock obviously knows it. "You manipulative bastard," he says.

Sherlock just grins, and waits for John to walk over to him. They duck under the lip of the door and into a brightly-illuminated warehouse packed with pallet racks stretching 10 meters up. Sherlock is already drawing out his wand as they enter.

John hears the scuff of feet to his left but he hasn't finished turning his head in that direction when he hears " _Expulso_!" and at the same time, from behind him- " _Protego majora_!"

A jet of brilliant red light rebounds off something invisible about three feet to John's left and angles back at the wizard who cast it, a dark-haired man in his mid-twenties wearing a black hoodie. He dives out of view and the red light hits a rack of pallets with the concussive roar of an explosion, obliterating three full pallets' worth of boxes and leaving massive black scorch marks across what remains.

Sherlock takes the shield charm down, then lopes off toward the corner that the wizard had fled around. John gets his back against the pallet racks and heads the other way. One of these days, he was going to learn to stop taking what Sherlock said at face value; he should have predicted that Sherlock was going to pull this kind of stunt and brought his gun with him. Not that he couldn't handle himself in hand to hand, but it was probably going to be an all wizard cast for this fight scene, and wands are a powerful advantage that John doesn't have.

John keeps low and moves lightly and quickly past the towering pallets to the next aisle, which is empty. On to the next. There's another man halfway up it, also casually dressed in Muggle clothes, but holding a wand and jogging away from John. There's a flash of a black coat at the other end of the aisle, and the thug lets loose a spell in his direction- evidently not as powerful as _expulso_ , since it merely scorches the boxes instead of obliterating them. John immediately breaks into a run under the cover of the noise, and before the man can turn to face him John sucker punches him in his lower back. He goes down as easily as that, and John kicks him in the chin so hard that it bounces the back of his head off the cement floor. Not exactly sporting, but it's hard to feel guilty about bludgeoning someone who just tried to kill Sherlock. John only pauses to make sure the man is still breathing before he moves on.

It occurs to him that this is a ridiculous place for this showdown: a maze of pallets and cranes, with who knows how many armed thugs wandering around. And somewhere in here is Masingberd, the man in charge, unless he takes this opportunity to do a runner. John decides to continue up along the wall of the building, thinking that running the perimeter will give him the best chance of finding Masingberd.

The next wizarding thug isn't looking away from John. He immediately flicks his wand at John and bellows, " _Duro_!" John dives forward, which scrapes the shit out of the palms of his hands but causes the spell to go over his head and strike somewhere behind him. He rolls sideways and gets to the man just in time to grab his wrist and jerk his hand up as he shouts the spell again. John feels a surge of power like a gust of wind ruffling his hair as it goes past his left ear. He keeps his grip on the man's wrist with his left hand and punches him in the face with his right.

The wizard claws at John's eyes with his free hand, but John grabs the wand above the wizard's hand and pushes as hard as he can, causing the wood to snap in two. The wizard roars incoherently and throws a wild haymaker, which John easily deflects. John responds with a series of forceful punches that pummel the man's midsection and leave him staggering backward. John grabs him by the front of the shirt, and as he scrabbles madly at John's arms and torso, John finishes by punching him in the face repeatedly until he drops to the ground, insensible.

John can hear shouted curses and counter-curses from somewhere off to his left. The echoes bouncing through the large space distort the voices terribly, but John can still distinguish Sherlock's rumbling baritone. Especially when it rises into a scream of absolute agony.

"Sherlock!" John hollers without thinking. He realizes dimly that he's given away his position, but it hardly matters. He abandons strategy and runs up the aisle toward the other side of the warehouse, the probable source of the shout.

"I'm fine!" Sherlock yells back, sounding rather strangled. "Don't come over here!"

John skids around the next corner and into view of two more wizards, both raising their wands. He ducks back into the aisle and hears their shouted spells rebound off the pallets with a series of splintering crashes. He can hear the footsteps of one still approaching, and sure enough when he runs back into their view, one of the men is only steps away from the corner. John grabs big fistfuls of his shirt and yanks the man after him behind the dubious protection of the pallets. John smashes him up against the nearest rank of boxes, then grabs his wand hand and twists until the bones of his wrist grate and he drops the wand with a shriek of pain. John twists the man's hands up behind him, and when he ducks back around the corner this time he's holding the wizard in front of him like a human shield. Fortunately the guy is even more compact than John himself, and skinny to boot; he's easy to man-handle ahead as John advances on the other wizard, a bigger chap who's wearing a button-down shirt and a completely befuddled expression.

" _Incarcerous_!" the man in the button-down shouts, just as John shoves the skinny man forward at him. Conjured ropes wrap themselves around the unlucky wizard, who falls face-first onto the ground. John tackles the wizard to the floor before he can cast again, and the man drops his wand when he’s hit with the force of John’s entire body. 

"John! Incoming!" Sherlock's voice shouts- closer this time, John thinks- and there's a virtual stampede of running footsteps as John pelts back in Sherlock's direction. A man with a vibrant flame tattoo just under his right eye hoves into sight: Masingberd. He's a little quicker on the uptake than his men, and shouts, " _Confringo_!" at John as he dashes straight at him. John jerks sideways out of the way of the spell. The bright burst of light and energy practically singes his arm, and John is knocked to the floor as something behind him explodes. Thinking quickly, John rolls back into Masingberd's path and snags his trouser leg as he runs by, bringing him to the floor.

It's a scrum then, both fighting to get up while pushing the other down. They roll to and fro on the floor, Masingberd kicking at John and trying to get his wand into play, and John trying to climb up Masingberd's body and get within punching range of his torso and face. There is a great deal of unsportsmanlike conduct, not to mention cursing. Masingberd manages to knee John in the jaw, hard enough to leave what will no doubt be a spectacular bruise, but John is giving as good as he gets with whatever fist isn't currently locked onto Masingberd's clothing.

Finally they both manage to struggle upright and push back from each other, staggering for footing. Masingberd immediately tries to run toward the loading dock, but John lunges forward and grabs him by the back of his shirt, jerking him to a halt. Masingberd abruptly steps backward and stomps hard on John's left foot. John bear-hugs the wizard around his midsection, bowing his head against the back of his shoulder to prevent Masingberd from smashing him in the face with the back of his head. John manages to lock Masingberd's arms up behind his back, and twists his right wrist until he drops his wand. John kicks it away.

"Filth!" Masingberd howls in outrage. "How dare you!"

"Stop fighting me, you've _lost_ ," John barks; it's taking most of his concentration to keep Masingberd from fighting loose from the armlock. He turns his head and sees Sherlock stalking up the aisle, looking very, very angry. "Finally," John says.

"We were warned someone was coming!" Masingberd snarls, and tries to stomp on John's foot again. John wrenches his wand arm higher behind his back, and Masingbird shrieks in pain.

Sherlock stops a few meters away; he has his body bladed with his right hand towards them, but John can tell from the way he holds his shoulders that something's wrong with his left arm. "Really?" he asks pleasantly. "What were you told?"

Masingberd laughs breathlessly. "They said Sherlock Holmes was looking for us! The consulting wizard and the Mudblood faggot he drags around on a leash!"

John doesn't react, he's been called worse by better. But Sherlock bares his teeth in an angry snarl. "Someone ought to teach you better manners." 

Masingberd sneers and hawks a gob of spit at Sherlock that lands on his trousers. "You? Or your _pet_ here?" The anger vanishes from Sherlock as if a switch has been flipped, and his expression goes completely blank.

John finds that he does not like the look on Sherlock's face at all. "Sherlock," he says warningly.

Sherlock blinks rapidly and the cold blankness goes away. "You can't afford our fees," he says flippantly.

There's a sudden shout from the direction of the loading dock. "Masingberd!" a voice roars. "This is the LEP! You and your men throw down your wands, and I'll make sure your term in Azkaban is nice and short!"

"Oh good," John says, deadpan. "It's the Ministry. We're saved." 

Sherlock looks furious rather than amused. "We need more time!" he says despairingly. "We didn't even get to question him. We don't know who he's working for, or what's next-"

"Oh, I'll give you that for free," Masingberd sneers. "I don't know his name, but he's a great man, I can tell you that. A proper wizard, with a proper view of things. He's going to put things right, he is, and fuckers like you lot will get what you deserve-"

"Sherlock," John says tensely. "Unless we want to get arrested, we don't really have any more time to waste listening to this idiot."

Sherlock frowns, but he also raises his wand and mutters, " _Stupefy_." It's rather sulky, as spells go, but it works well enough and Masingberd goes limp. John lets him slide to the floor and shakes out his own arms before turning fully toward Sherlock.

"Holy Christ, what did you do to your _arm_?" John demands.

Sherlock glances down and looks surprised, as if he has forgotten that his left arm is dangling uselessly at his side with part of the humerus actually protruding and the sleeve entirely soaked in blood. "Oh," he says. "That, yes."

"We're coming in!" someone bellows from the loading dock.

"Morons," Sherlock grumbles. He grabs John’s sleeve with his right hand and narrows his eyes in concentration. “Damn it,” he says after a couple seconds. “Anti-disapparition Jinx.”

“Side door, the one we broke in through the other day,” John says. “Come on.”

Miraculously, the door they find their way out through is unguarded. Sherlock mutters something vicious about LEP's hiring standards and starts looking for a cab. "Baker Street," he says once they're inside.

"You sure?" John says doubtfully. A hospital might be better, with the way Sherlock is bleeding everywhere; from the rate it's not arterial, but they need to get the bone set in order for John to put pressure on to stop the bleeding. Even if Sherlock hates St. Mungo's casualty department, he can always get the arm fixed the Muggle way and then go get it spelled later. John tries to remember what medical and magical supplies they have back at the flat, but Sherlock cycles through potions and ingredients so fast it's impossible to keep track.

Sherlock had tucked his wand away as soon as they left the warehouse, for appearance's sake, but his fingers are flicking nervously over its outline. He takes out his mobile and immediately puts it away again. John imagines that the pain in Sherlock's arm must be incredible, and is somewhat impressed that he isn't screaming his head off right now. In fact- John reaches over and checks Sherlock's pulse with one hand, tilting his head with the other. His pulse is weak and thready, his pupils enormous, and his skin is cold and clammy despite the flush in his cheeks.

"You ass," John says. "You're in hypovolaemic shock."

"Am I?" Sherlock says distractedly.

"We need a hospital," John says.

"No," Sherlock says vehemently. "Baker Street."

John bites his lip and glances out the window. They're halfway home already, and if they go home John will not have to think up an explanation for Sherlock's injury that doesn't involve wizards. He wonders if the MLEP will check the local hospitals for injured people, the way the Met would after a major fight where an injured party escaped.

"They will," Sherlock says. "They're stupid, but regrettably not that stupid." John is still torn. "There's a bottle of Bone Glue under the sink in the loo," he says, and that decides it.

"Baker Street," John finally concedes.

"You can fix it," Sherlock says. "You're a doctor." Sherlock snorts with laughter, and John joins him. It tends to be like this after they're in danger; the adrenaline high leading to near-euphoria expressed in wildly inappropriate laughter. 

When they reach home, John tips several extra bills into the cabbie's lap to apologize for the blood on the seat which he hasn't noticed yet, and practically shoves Sherlock up the stairs into the flat. He stands in the middle of the room, swaying slightly. "Sit down on the sofa," John says and rushes for the loo to rummage through the myriad of bottles under the sink. The Bone Glue's trademark blue flask is crammed all the way in the back, wedged under the u-bend of the sink's drain. Unlike Bone Glue, the recipe for Blood-Replenishing Potion is non-proprietary and appears in any basic healing or potions handbook. Sherlock brewed up a batch just the previous week, and so there are several liter bottles with homemade labels full of the stuff. John grabs one along with another bottle labeled "Wound-Cleaning Potion" and abandons the bathroom.

John detours to his room for his "just in case" bottle of oxycodone and returns to the sitting room a minute later, after a brief stop in the kitchen for a glass of tap water. Sherlock is sitting on the couch, partially slumped over on his right side. John perches on the edge of the coffee table and dumps the potions down next to him. "Sherlock," he says, and shows Sherlock the bottle of painkillers. "You may have one of these, if you want it."

"Yes," Sherlock says immediately, almost whispers. John opens the bottle and pops the pill into Sherlock's mouth, then puts the glass to his lips. He thirstily gulps the water down, and John glugs the glass full of the red, viscous Blood-Replenishing Potion. Sherlock curls his lip in distaste; he always complains that the stuff puts him off because it makes him feel like a vampire. John doesn't mind it, but then he doesn't end up drinking it as often as Sherlock. "That much?" he asks dispiritedly.

"All of it," John says firmly. "Or it's the hospital for you. Maybe I'll just roll you out of the cab next to the A&E and let you explain it all by yourself."

"You would never," Sherlock says, but he obediently slugs the stuff down when John puts the glass to his lips. He makes a face. "More water?"

John obediently goes to the kitchen for another glassful. Sherlock takes it away from him when John tries to hold it to his lips. "Stay here and don't try to do anything," John directs. "Give the oxycodone a minute to kick in, because the bottle says this causes 'some discomfort,' which in my medical experience means it's going to hurt like a bastard."

John scoops up the Wound-Cleaning Potion and the bottle of pills, and takes them out to the kitchen, where he gets out a clean measuring cup and pours it half full of potion. He gets the supplementary first aid kit out from under the sink- they practically have one in every room at this point, but John has learned that's just good sense in any flat where Sherlock resides- and pokes through it to find cotton and a syringe. John was never an abuser of substances of any class, but when one has been to medical school- where a certain percentage of overworked students feel the pull of artificial stimulants- one picks up a few things. It takes John about five minutes to turn two of his oxycodone into a thin golden liquid that he squirts into the cup of Wound-Cleaning Potion and stirs with a clean spoon.

He carries it out to the sitting room, where Sherlock is still hunched over on the sofa looking miserable. "Wound-Cleaner first?" Sherlock says doubtfully, eying the cup of purple liquid. "Shouldn't that come after the-"

"Doctor Watson says no," John says briskly, digging a pair of scissors out of the top drawer of Sherlock's desk. Sherlock scowls when John rends the fabric of his jacket and shirt to better expose the wound, and flinches when he peels it away from the skin, but doesn't say anything.

At least two inches of glistening bone are jutting up from Sherlock's outer arm, and a wide, messily-torn gap in the skin has filled up with blood. John doesn't worry about the blood, just picks up the measuring cup and starts to liberally pour the contents over Sherlock's arm, coating the entire wound. The potion hisses and smokes on contact, but Sherlock only flinches a bit. When the cup is empty, John takes it into the kitchen and rinses it in the sink, to give the potion a minute to work. He returns a moment later, opens the blue bottle, and begins the pour the Bone Glue directly onto the exposed bone. Just a dribble at first, and the stuff hisses and spits rather alarmingly as it strikes the protruding bone. The smoke the working of the potion produces on contact is blue-gray for some reason. John clinically adjusts the stream of the pour. Per the instructions, he stops and caps the bottle when the hissing stops and the bone visibly starts to shift.

The bone slips back through the hole in Sherlock's arm and slots itself back into place. John isn't sure at first if the grating he can hear is the sound of the bone realigning, or of Sherlock's teeth grinding. But when he glances up at Sherlock's face, he looks fairly detached, craning his neck to watch the potion work with every indication of keen- if slightly drugged- interest.

"All right?" John asks when the potion seems to have ceased its effect.

Sherlock nods, slowly, and looks up at John. "What did you do?" he asks. "I barely felt that, and one pill of time-released oxycodone shouldn't affect me that much. You did something to one of the potions. Not the Bone Glue or the Blood-Replenishing, you opened those in front of me. The Wound-Cleaner, you took it in the kitchen. What did you do?" It's an unusually slow stream of deduction, the euphoria from the narcotic must be kicking in.

"I cooked down 60 milligrams of oxycodone and added it to the Wound-Cleaner," John says.

"Ah," Sherlock says. "Topical anesthetic. Why didn't I think of that?" He sounds a bit put out, as though he ought to be the first to think up any advancement involving chemistry or potions. "You don't worry about rapid absorption?" John opens his mouth to explain, but he doesn't get a chance because Sherlock's eyes immediately widen and he answers his own question. "No! The oxycodone binds to the- Oh, that's brilliant, it completely eliminates the risk of systemic toxicity." Sherlock tries to lurch upright, and John grabs him by the sleeve of his uninjured arm, because he seems ready to jump up and run into the kitchen to duplicate John's work.

"Don't get up, for Christ's sake, Sherlock. You still need to finish the job," John says, gesturing to the gash still marring Sherlock's shoulder. "Unless you want me to stitch you up."

"Oh, I'll fix it in a minute," Sherlock says impatiently. His pupils have retracted severely, which accounts for at least some of this behavior. "How did you think of it, John? When?"

"My mum used to send me potions and things all the time," John explains. "In medical school, and...later."

Sherlock studies John's face. "She's a Healer."

"Yeah," John says. "It was also kind of a jab, you know- Muggle doctors are rubbish, make sure you have some _proper_ medicine on hand. But it was a shame to waste it, and sometimes I'd...experiment." John grins sheepishly and rubs the back of his neck. "I got in some trouble, a couple times. Had the Ministry on me for exposing Muggles to potions and the like. But I couldn't- not when it could help people." They threatened to haul him out of Afghanistan and straight to the Ministry for a full-on trial, if he didn't stop saving Muggles' lives with what was technically considered healing magic. "Someone told my mum, then, and she stopped sending me things. Couldn't brew the potions myself of course." Not without a wand.

Sherlock is staring at him in a very strange way, as if he is both wonderful and deeply interesting. It's a look John has seen several times before: typically on the faces of patients who have just awoken after surgery and are visibly putting together the pieces and concluding _this is the doctor that saved my life_. Most recently, John has seen it on Sherlock's face, on the second night they met, right after John shot that serial murderer. For just a moment he is not just the focal point of Sherlock's observations, he is the only thing in Sherlock's world at all, and it feels incredible. John wonders if this is how Sherlock feels whenever John tells him one of his deductions is brilliant, and he can see why Sherlock might be addicted to this feeling.

The moment passes as the doctor in John takes over and he leans over to pull the wand out of Sherlock's jacket and poke his right hand with it. "Finish," he says firmly.

Sherlock obediently grasps the wand and angles it so he can awkwardly wave it at his own upper left arm. Luckily, he is reasonably proficient at healing spells even in a drugged stupor, and quickly manages to close up the wound. He staggers to his feet and nearly trips over the coffee table.

"Where are you going?" John says. "Sit down, you lunatic, before you fall down."

Sherlock sways slightly. "Change," he says. "No more spells. Too tired."

"I can get you another shirt," John protests, grabbing ineffectually at Sherlock's sleeve as his hands are swatted away.

" _No_ ," Sherlock says vehemently.

"Fine, God," John says, resigned. "At least let me help you." Sherlock consents to this, and John half-supports his flatmate as he staggers into his bedroom, which is crisscrossed by hanging wires strung with amulets and Dark Detectors. The floor is cluttered with the same heaps of books and random experimental apparatus that predominate in the sitting room. The overall effect is of some kind of extremely paranoid wizarding street bazaar. John allows himself to be shooed outside and Sherlock slams the door rather abruptly in his face. There are several minutes of rustling and muttering, followed by an alarming silence. John raps lightly on the doorframe with his knuckles. "Sherlock?" he says. "If you have passed out on the floor, I'm going to be extremely cross."

"Come in," Sherlock says. When John does, he finds Sherlock sprawled across the bed on his right side, with his left arm curled over his stomach. He has discarded his ripped and bloodied shirt and jacket on the floor, and somehow struggled into a t-shirt and his blue dressing gown, which looks extremely bizarre over his suit trousers and the very nice loafers he is still wearing.

"How do you feel?" John asks. He leans over to check Sherlock's pulse, which seems to be back to normal, as does his breathing. The Blood-Replenishing Potion has done its work then, and John's foray into pharmacology hasn't caused any harm either.

"Very...fluid." Sherlock flaps one hand at him vaguely.

"You look insane," John notes. He stoops over to slip off Sherlock's shoes. "But for the record, I draw the line at helping you change your trousers."

"Arm's sore," Sherlock complains. "I deserve sympathy." He's slurring his words rather badly.

"I may make an exception," John says. "But only in a circumstance where your arm comes off entirely."

"Get a prosthetic," Sherlock muses. "Enchant it."

"Let's hope it doesn't become necessary, right?" John pats Sherlock's leg and stands up. "Get some rest." Sherlock closes his eyes.

John closes the door behind him, and goes back to the sitting room to tidy away all the bottles so he can get some sleep himself.

* * *

When John awakes the next morning, Sherlock is already up and dressed in a fresh suit. He is typing away at John's laptop- a fact which John magnanimously pretends not to notice- from his usual perch on the sofa. John gives him a friendly 'good morning' and goes to make the tea. He's just returning with the mugs when he hears the characteristic crack and whoosh of someone apparating downstairs. "Mrs. Hudson?" he speculates.

"Too much mass," Sherlock notes, closing the laptop.

"Bloody wizards," John mutters, setting one mug in front of Sherlock. Why couldn't they just use the front door like normal people?

"Laziness," Sherlock says, as if reading his thoughts. "They can't apparate on the street, and if they can't apparate somewhere, they won't bother."

"You're a wizard, and you never apparate anywhere," John says.

"I'm not lazy," Sherlock says. "Ah, Dye, thank you for proving my point." Howard Dye has indeed appeared in the doorway, wearing black robes over a black suit. John stifles a laugh at Sherlock's acerbic comment and thinks that the Aurors' office parties must be terribly grim. John has known a lot of people who fight for a living, but most of them at least have a sense of humor.

"Holmes," Dye says in a flat voice. He looks directly at John. "I see you haven't taken my advice. Pity."

John raises an eyebrow. "I don't recall ever _asking_ for your advice," he notes.

"Well done," Sherlock says. "Aurors are not especially bright to begin with, and Dye is rather stupider than average."

Dye scowls and takes out his wand, just slowly and casually enough that John does not interpret it as a threat, although he does sidle a little closer to the wizard. " _Accio_ basilisk venom!" Dye declares, waving the wand. Nothing happens. Nothing continues to happen as Dye stalks into the kitchen and repeats the procedure, and John risks a glance at Sherlock. Sherlock smiles beatifically at John, which causes him to unknot a bit. Either Sherlock has gotten rid of the seized venom, or he has very skillfully hidden it where a summoning charm can't fetch it. Oh god, what if it's up in John's room? It had better not be in John's room.

"Another well-timed illustration," Sherlock notes from his place on the sofa.

Dye turns on him, scowling and clenching his fingers tight around his wand. "I need to see your wand, Holmes," he snaps.

Sherlock steeples his fingers below his chin. "This is becoming rather a habit of yours," he notes. "Am I to expect one of these little visits every time the Aurors are stumped by a case? That would infringe considerably on my time."

"Your wand," Dye repeats, holding out his hand pointedly.

Sherlock continues to regard the Auror with an air of detachment. "I think...no. I am not feeling especially cooperative today." He breaks his pose and stretches his arms over his head, cracking his knuckles. Then he picks up his wand, which is still sitting on the coffee table where he dropped it last night after healing his arm.

"I have a warrant," Dye says. He smiles and pulls a folded paper from his pocket. Sherlock makes no move to approach, so John takes two steps closer and puts out his hand for the paper. Dye lets him take it.

John scans the warrant, which was evidently written up by Dye himself. It talks about the basilisk venom, the warehouse full of bullets and the injured Masingberd and his cronies. There's a Wizengamut stamp on the bottom, and a signature and counter-signature. It looks real enough to John, but he's not really in a position to know. He offers the warrant to Sherlock, who waves him off and tosses the wand at Dye.

Dye fumbles a bit as he catches it. Good reflexes, but clumsy. He holds Sherlock's wand up with his left hand, and casts with his right. " _Prior incantato_!" The stream of smoke the wand emits this time is a periwinkle blue, with a few wisps of white. "Vulnera sanentur," Dye says.

"So it seems," Sherlock agrees.

"That's a powerful healing spell," Dye says. "Mind telling me why you had occasion to use it? Especially with a-" slight sneer, "doctor in residence?" John grits his teeth; that was just unnecessarily snide. He had to go through more training for his profession than Dye did for his, he doesn't think it's too much to ask to be accorded some respect for it. Or at least to not be sneered at.

Sherlock glances sideways at John and he seems to develop a little sneer of his own when he answers, "I cut myself shaving."

It's such an obvious and bold lie that John almost laughs. The look on Dye's face is extremely sour and John can see why. The warrant proves they suspect Sherlock of being connected to the fight in the warehouse, which makes the healing spell cast by his wand suggestive. But it's not enough evidence to definitively place Sherlock at the scene. "I see," Dye says through gritted teeth.

"Do you?" Sherlock says with every appearance of honest curiosity. "What a pity that your office thinks that forensic spellwork begins and ends with that spell." He holds his hand out expectantly. "If you're quite done."

Dye scowls at Sherlock's wand as if it's personally affronted him, and flings it back onto the coffee table. Sherlock reopens his laptop and takes a sip of tea, making a face. "John, the tea's gone cold," he complains.

John tests his own. "It's still warm," he contradicts. "If you want more you can make it your bloody self." Sherlock sniffs in displeasure and goes back to typing. A quick glance at Dye shows that his face is turning slowly red.

"This isn't the end of the matter, Holmes," Dye says.

Under the circumstances, Dye's severe tone is almost comical. John takes his tea over to his armchair and picks up yesterday's Daily Prophet. "There's an article in here about neo-Voldians," John notes.

"I don't care," Sherlock says, still typing. "If I were interested in that drivel I am perfectly capable of reading it myself."

"Yes, but would you bother?" John says amiably, and flicks over to the next page. Dye just stands there getting more and more irritated until finally he stomps out the door and down the stairs. John manages to stop himself giggling until after he hears Dye disapparate, but it's a near thing. "Oh lord, his face. Nicely done."

"It's barely even sporting," Sherlock says, but he's smirking all the same. "It's like matching wits with Anderson." 

"Good thing there isn't a spell that can tell what else you've cast on your wand," John says, calming down.

"Of course there is," Sherlock says.

"What?" John asks, startled.

Sherlock smiles faintly. "As I said. The Ministry thinks forensic spellwork begins and ends with priore incantatem."

John casts him a skeptical look; not necessarily because he disbelieves there's a spell that Sherlock knows and the Aurors don't, but because it's the quickest way to get a rise out of him. "Oh?"

"Shut up!" Sherlock retorts. He digs around under the sofa cushions until he produces another wand- darker wood, much shorter than his own. John is going to have to search that sofa later on; does Sherlock have some sort of wand repository in there? He flings the spare wand down on the table and raises his own. " _Omnes incantato_!" he snaps, gesturing at the spare. The stubby wand spits out curls of green, then a large puff of blue, spurts of mauve, quite a long strand of ochre- color after color, breath after breath of mist, spins off the end of the wand and hangs in midair until the air between John and Sherlock is thick with multicolored smoke. " _Finite_ ," Sherlock drawls, and as the mist vaporizes John can see that he looks impossibly smug.

"Yes, all right," John concedes. He returns to his chair and his rapidly cooling mug of tea, examining the warrant again. "This is odd," he says. "They wrote that your name was painted on the floor of the building."

"In yellow," Sherlock says. John's startled look is all the answer he needs. "Careless of me. I saw the paint on the floor, should have wondered more about it at the time, but we were rather busy. It wasn't discernible as letters from the ground, they must have had to get to the top of the pallets to see it. Is there more than just my name?"

"Just some gibberish," John says, rereading the relevant line in the warrant. "Sherlock, why would someone write your name on the floor of that building?"

"Presumably because they expected me to turn up," Sherlock says. "Remember that Masingberd was alert to our arrival. _They said Sherlock Holmes was looking for us_ , he said. Let me see the warrant." 

John holds it out but Sherlock makes no move to retrieve it; John sighs and gets up to go over and put it in Sherlock's hand. "Remind me to laugh next time you tell me how lazy you're not," he says. Sherlock ignores him in favor of digging a marking pen and an old printout from Google maps out of the heap of papers on the coffee table. He flips the map and carefully copies the string of letters and numbers onto the back, then caps the marker and sets the warrant down.

"Interesting," Sherlock says thoughtfully. "They spelled my name with two e's: Sherleck. I wonder why?"

"The rest of the message is gibberish," John points out. "Maybe none of it means anything."

Sherlock dismisses this with a wave. "No, it's my name; nothing else it could be. This is clearly a cipher. More tea, John. This may take a while."

In point of fact it takes only fourteen minutes before Sherlock sits back and makes a disgusted noise, pushing away the cipher along with another scrap sheet on which he's scrawled the alphabet over and over in a large block of text, shifting one letter to the right on each row. "Vigenére cipher, with the only unencrypted word- my misspelled name- as the key," he says. "Juvenile. He needn't have even bothered."

"What's it say?" John asks. Sherlock flips the scrap paper around so John can see his translated text: _Great spirits have always encountered violent opposition from mediocre minds_. "Is that a quote or something?"

Sherlock drums the fingers of his wand hand on his jaw. "Yes," he says. "But I can't remember what from." He turns to John's laptop and pulls up Wikiquote. "Albert Einstein," he says. "Hmm."

John blinks, taken a bit aback. "Somebody made up a cipher and painted it on the floor in three foot high letters so they could quote Albert Einstein at you?" he says. "Who the hell _does_ that?"

"Someone who wants to get my attention," Sherlock says, spinning the paper back around and studying his translation. "The quote goes on, 'The mediocre mind is incapable of understanding the man who refuses to bow blindly to conventional prejudices and chooses instead to express his opinions courageously and honestly.' It almost fits me, wouldn't you say? The man who speaks honestly despite the press of convention against my sort of reasoning?"

"I'm not sure I'd describe your willingness to blurt out people's personal lives in public as 'courageous' so much as 'insufferably rude,'" John says drily.

Sherlock merely casts his eyes up at the ceiling and leans back on the sofa into his Thinking Pose, hands beneath chin, wand between palms. "Albert Einstein is best known for his theories on relativity, but he also laid the groundwork for the development of quantum theory, among other subsequent discoveries. His work was almost entirely theoretical, rather than practical, but he is remembered today as the archetype of the mad scientist and as one of the greatest geniuses in modern history."

"I'm surprised you know all this," John says. "I would have thought you'd deleted it, along with the history of magical law and who won the last Quidditch Cup."

"I know it because physics is _relevant_ , unlike the rest of your trivia. Shut up," Sherlock snaps. He pauses for a moment. "The theory of relativity was postulated in 1905, but it remained controversial for some years," he says thoughtfully. Sherlock slowly rolls the wand between his hands. "The writer of that cipher may have been referring to me, or to himself, or to both of us. If he's referring to himself, he's suggesting that he considers himself to be a genius and his work to be extremely significant. He's also suggesting that only a mediocre mind would object to what he is doing. I need more data."

"And how do we get that?" John asks patiently.

"We answer in kind," Sherlock says thoughtfully. "But why did he spell my name wrong?"

"Did you try running it through the cipher with the same key?" John asks. "Maybe it spells something else."

Sherlock sits up and glances at his alphabet table again. "Doesn't work," he says. "Gibberish. But- oh! Oh, John, you're _brilliant_." He picks up the marker and begins to scribble a new chart, under which he scrawls _Einstein_ , then _Sherleck_. His lips move silently for about a minute and a half, and then he begins to rapidly mark down new letters.

M-o-r-i-a-r-t-y.

"What the hell is that?" John asks, when Sherlock sits back and caps the pen.

"It looks to me," Sherlock says with great satisfaction, "Like a name."

* * *

"I would like to reiterate," John says as he peers over the edge of the roof, keeping an eye out for the constable walking the perimeter of the warehouse, "that I oppose this idea entirely on the grounds that it is insane."

"Yet here you are," Sherlock says back, shaking a can of "Tibetan Blue" paint, which he insisted would show up better against the brickwork than plain white. He is standing around the corner from John, on a narrow ledge, preparing to paint across the gable so the words will be visible from the street.

"It's even more insane if you don't have someone to watch your back," John says, and then thinks about what he is saying. "My God, I'm enabling you, aren't I? I've heard about this, the cycle of enabling. Following you around, ignoring all the stupid shit you do- that’s how it starts."

Sherlock aims the nozzle at the brick and carefully forms a large upper-case A. "Shut up, John."

"I can’t remember what comes after that. Legal problems? Both of us getting arrested _would_ be the next logical step," John says.

Sherlock finishes the first word and moves to the second. "You can always try Co-Dependants Anonymous."

John squints, wondering if that's actually a real thing. "Are you trying to get rid of me?"

"I am trying to shut you up. This is rather difficult, and street art is not one of my areas of expertise."

"Maybe the cycle ends with me pushing you off the roof," John suggests, but he shuts up, if only because he doesn’t actually want Sherlock to wind up splattered on the street four stories below. It's fifteen more agonizing minutes- with one pause to wait while the constable strides into and out of view of the gable- before Sherlock caps his can of paint and declares himself done. "Good," John says. "Now let's go home, please."

Sherlock smirks. "Don't you want to admire my work?" He gestures grandly at the wall.

 _Any intelligent fool can make things bigger, more complex, and more violent_ , John reads. Good sentiment, although he wonders how well this Moriarty character will react to being called a fool. "No cipher?" he asks.

"No point. He'll know I cracked his, and he'll know it's from me." Sherlock lets John give him a hand to brace against as he swings his leg up onto the roof and pulls himself up with hands that are stained blue.

"Yes, it's both direct and insulting. Who else could it be from?" John says.

Sherlock's mobile starts ringing while they are secreted in a nearby alley cleaning the paint off Sherlock's hands. He gives John a meaningful look and John sighs and fishes the phone out of Sherlock's inside jacket pocket with an ease borne of frequent repetition. The incoming call is from "The Government."

"It's Mycroft," John says.

"Ignore it," Sherlock directs, and John shrugs and sticks the phone back in Sherlock's pocket. His own phone goes off then. "That's him, too," Sherlock says. "Ignore it."

John pulls his phone out, and sure enough the caller is listed as "Unknown." He looks at it for a moment, considering what the next step might be if he refuses to answer. Mycroft can be annoying persistent.

"John," Sherlock says urgently. "Damn it!" He can obviously tell what John is thinking, but his hands are still occupied.

"Hello," John says.

"I sincerely hope that my brother knows what he is doing," Mycroft says in a tone of voice which suggests that he is certain Sherlock has no idea. It is very much an older sibling tone of voice.

"Is that all?" John asks. "Or did you have some pearls of wisdom to impart?" He is feeling punchy, and he likes the approving smirk that Sherlock shoots him when he cheeks Mycroft.

"Just show him your mobile," Mycroft says, and hangs up. A second later the phone pings with the warning of a new text message. John opens it up and sees, _The world is in greater peril from those who tolerate or encourage evil than from those who actually commit it._ John shows the phone to Sherlock, who merely snorts.

"Einstein," he says. "Speaking about the virtue of standing on one's principles rather than making deals with the devil. Trust Mycroft to take the quote _entirely_ out of context."

John looks at the message again. "Is he suggesting that you are more dangerous than Moriarty? Is that a compliment or an insult?"

"From Mycroft? Both." Sherlock finishes cleaning his hands. "Let's walk by the Tube station, I need to send out a message."

John tags along to the nearest Tube station, where Sherlock unerringly tracks the wailing of a saxophone to the edge of the platform. A too-skinny teenager in ratty trainers with the pale, gaunt, well-lined look of the addict is busking with half of a styrofoam take-out container in front of him, weighted down with change and a few bank notes. Sherlock doesn't even make eye contact as he passes the boy and lets fall the train schedule in which he has jotted notes and tucked several bills of his own. They take the Tube home as if that is what they had intended all along.

There are no cases for the next couple days, and no further visits from the Aurors. Sherlock remains strangely un-bored. The excitement of this new game, waiting for a return message from the mysterious Moriarty, keeps his mind occupied even though he has no actual work to do. John is rather more on edge, feeling as if he is waiting for the other shoe to drop. It's unclear what Moriarty may be after, or whether he's actually connected to the schemes that Sherlock and John have uncovered. If he is, however, he's probably the most dangerous lunatic in London right now. And he is clearly very interested in Sherlock Holmes.

John isn't sure whether he's relieved or terrified when a waif-ish pickpocket outside the chip shop on Marylebone tucks a piece of paper into Sherlock's pocket when they're picking up a late dinner. On it is written a familiar address which turns out to be the home of Christopher Ames. John doesn't recognize the name at first, but when they arrive he immediately recognizes the home of the third chemical plant murder victim: the first crime scene Sherlock had taken him to.

"He's revisiting the sites of his previous crimes," Sherlock says. "Taking credit."

"Why?" John asks, but Sherlock doesn't answer. They enter the house the same way the message-writer evidently did, through a broken-open door in the back. The house has been standing empty in the months since its owner was murdered, and the door had been boarded up at some point. The message, again in cipher, is painted in small, neat letters on the kitchen cabinet against which Ames' body was found.

Sherlock pulls out his notebook then and there and works out the cipher. It is the same type as before, this time using Moriarty as the key. Sherlock tsks disapprovingly. "Predictable," he says. "Disappointingly so."

"Yet it's still unclear what he's doing, or why," John replies.

"Yes. Which suggests the predictability is a front." Sherlock pockets his pen. "He's being predictable because he does not want me to learn about him from these messages." He shows John the notebook, with its decoded message: _The secret to creativity is knowing how to hide your sources._ Sherlock frowns. He seems unaccountably irritated. "Or at least, he does not want me to learn anything he has not specifically intended. Let's go out front," he says.

Even knowing Sherlock's habitual recklessness, it shocks John somewhat when he pulls out a can of spray paint as they stand on the street in front of Ames' former home. John grabs his arm and darts his eyes frantically around, but there seems to be no one to witness the indiscretion. "Are you mad?" he demands. This is a residential neighborhood, someone will see!"

"Hardly," Sherlock says, shaking off John's grip. "The houses are all dark. It's nearly midnight. This is not a through road, so there's virtually no traffic. I don't want to have to wonder if some lackey has come back and spotted my message." He shakes the can and calmly, efficiently- has he been practicing his can-handling? It's better than last time- writes his reply on the sidewalk in letters two feet high.

_Sometimes one pays most for the things one gets for nothing._

The first interchange was fairly clear, but this leaves John mystified. "Mind explaining?" John asks as they head for Baker Street. "It's Einstein again, I assume."

"Obviously," Sherlock says. "The man is a thief. He fancies himself a genius, but he steals design documents from chemical plants. So where is his creativity?"

"What about the bullets? You said that was brilliant," John says. He's still rather perturbed by that, Sherlock's apparent admiration for war criminals, and he's not about to stop reminding Sherlock how very _not good_ that was.

"I said the application of the venom to Muggle technology was brilliant," Sherlock acknowledges. "But the synthesis of the venom-" He stops abruptly. "Never mind."

The obvious evasion so irritates John that when Sherlock's phone rings an hour later with another call from The Government, John picks up and puts it on speaker before Sherlock can even reach for the end table where the phone rests.

"I think you should know," Mycroft says, "That the Ministry has no record of anyone called Moriarty. Nor does any other branch of Her Majesty's government."

"Obviously I had already determined that," Sherlock says. "But thank you for once again providing evidence of your penchant for spying."

"It was a very juvenile cipher," Mycroft says, and hangs up.

"Had you really?" John asks Sherlock.

Sherlock, still irritable, glares at John. "I know you believe I'm reckless," he says. "But at least do me the courtesy of remembering that I'm a genius. I did make inquiries, which I didn't mention because there were no results."

"So Moriarty is- what? A fake name, another cipher?"

"It doesn't matter," Sherlock says. "It's unlikely he created the name purely for this set of messages. The fact that there are no references to it anywhere indicates that the man has significant resources. That's the real point."

* * *

They don't need homeless network intel to spot the next message: Moriarty paints it on the floor of Gilles Lestrade's office, and the DI turns up at Baker Street in a fury.

"It's addressed to you," he snaps. slapping down a photograph in front of Sherlock. John cranes his neck to look, and sees that it is in fact addressed to Sherlock, spelled correctly this time, but the rest of the message is the usual jumble of cipher letters.

"So obviously I didn't write it," Sherlock says. "I know it's difficult for you, Lestrade, but do try to use your brain."

"Two days ago there was a disturbance at 86 Sudbourne Road," Lestrade says. "You remember that address?"

"I have an eidetic memory," Sherlock says absently, copying the cipher onto a scrap sheet of paper. "Don't insult me."

"There was graffiti written in the street outside, and another of these ciphers inside- painted against the cabinet where Christopher Ames' body was found."

"And?" Sherlock prompts, not looking up.

"And that's a crime scene I brought you in on!" Lestrade says, snatching the photo back up. "I won't believe that's a coincidence, Sherlock. You're playing some kind of game here, and I won't have it. I want to know who got inside the Yard and painted this message without being spotted by a single constable or security camera."

"Your faith in my omniscience is gratifying," Sherlock says.

Lestrade looks like he's about to explode. "We don't know who it is," John says hastily. Sherlock shoots him a glare, which he ignores. "We're working on it, all right?"

"So tell me what you've got," Lestrade says. He waits patiently for a moment, but Sherlock seems prepared to let him go on waiting forever. "Sherlock!" Ultimately, Sherlock's willingness to sit in total silence outlasts Lestrade's willingness to wait around for an answer. “Fine,” Lestrade says. “Stonewall me, and we'll see what happens next time you're looking for a case to occupy you.”

"Well?" John says the moment Lestrade is outside the building.

"Well what."

"Well, what does it say?"

"Assuming it's yet another Vigenére cipher, the key word is different than either of the previous messages," Sherlock says. John takes it that this is his roundabout way of admitting he doesn't have any idea what it says.

"They key word could be anything, couldn't it?" John asks.

"Not anything," Sherlock says. "The first time it was my name. The second time it was the source of the quotation. Both times the key word could be inferred from the information contained in the messages themselves. He wants me to see what he has to say, he wouldn't leave me without a way to figure out the key."

"The last message wasn't addressed and didn't have a signature," John says.

"No," Sherlock agrees.

"It was something about sources," John mutters. Then, remembering, "The secret to creativity is knowing how to hide your sources. Right?"

"Yes," Sherlock mutters. "A key, a single word, something obvious in the context of his message but not necessarily contained in the text. He thinks he knows me enough to predict my thought process, so where does he think my mind would go upon seeing-" Sherlock's eyes and mouth grow slowly, comically wider. "Oh!" John's pulse jumps the way it always does when Sherlock has one of these revelations; like sympathetic pain juicing up his adrenaline. Sympathetic epiphany. Sherlock scribbles out a letter grid on the scrap paper. It is wider than the last few, a longer word.

John cranes his neck again to see what Sherlock is writing in his messy scrawl, and his heart almost seizes when he sees the word Sherlock has written as the key. "Voldemort?" John asks. He's never been afraid to say it, not since he was a boy, but he doesn't _like_ to say it. The name is thick and foul and heavy on his tongue.

Sherlock doesn't answer, just keeps scribbling his grid. He immediately begins to translate the message, and in less than a minute he sits back and flings down his pen. "It works," John says. "You got the plain text. How the _hell_ did you know that would be the right key word?"

A look of discomfort flits across Sherlock's face, which strikes John as very odd. This is perhaps the first time since they met that John has seen Sherlock hesitate to explain one of his deductions, and the first time he's done it without any show of pride in his own cleverness. "I'm beginning to get a sense of how this Moriarty thinks," he says. "A man who mixes Muggle technology with magic, but isn't afraid to delve into neo-Voldian circles for his henchmen. 'A proper wizard,' Masingberd called him. A man with a great deal of disdain for most of humanity. It's a logical inference that Voldemort is someone who preys heavily on his mind."

"To have security against atomic bombs and against the other biological weapons, we have to prevent war, for if we cannot prevent war every nation will use every means that is at their disposal," John reads. "Not quite as pithy as the others."

Sherlock bites his lip and drums his fingers against his thigh. "During World War Two, Albert Einstein helped draft a letter which urged the Americans to outpace the Germans in nuclear weapon development; this advice was followed, and Einstein later regretted that letter because he felt at least partially responsible for the use of these weapons at the end of world war 2."

"Okay," John says slowly. "So Einstein was against atom bombs. And against war, judging by the quote."

"Obviously," Sherlock says, with only a shadow of his usual sarcasm. "And Moriarty is manifestly not. He wants us to take something quite different from this message." He lays one long finger on the second half of the sentence. "Every nation will use every means that is at their disposal."

"Such as envenomed bullets," John says, feeling sick.

"And bombs that aerolize potions," Sherlock says.

"Oh fuck," John says. "You think he's going to test the bomb?" John's stomach roils. "But why is he telling us?"

"Why is he leaving me any of these message?" Sherlock says. "He wants to talk, he wants to show off. He wants-" Another expression of epiphany. "He wants me to see it. To figure out where he's going to strike." Sherlock clasps his hands under his chin, so tightly that the knuckles are practically white with it. "Choosing locations that are significant to his activities, to the chemical plant murders and the bullet production. Scotland Yard was an anomaly- not directly linked. He wanted me to get the message immediately. Time sensitive. It means he's moving into position now. Tonight, possibly."

"Tonight?" John feels like an echo chamber. "How the hell are we-"

"Shut up!" Sherlock barks. "I'm _thinking_. Significant locations. Moving backwards- warehouse, Ames, Yard. Or the Yard could be on the other end of the sequence, since the Yard is involved again now. Circular. Two more murder scenes before Ames, two after." He suddenly leaps into motion, seizing both their coats and tossing John's to him. "Get your gun. We'll have to split up." He digs his wand out from the debris on the kitchen table.

"That's not a good idea," John says. "Given that you ended up in mortal peril last time we split up."

Sherlock grimaces at him. "It's senseless to argue. We have four locations to check. When one of us finds the right one, he simply calls the other. There will be no mortal peril."

"Why is it down to us?" John demands. "We're talking about a terrorist attack- either the Yard or the Ministry would want to be involved. It's irresponsible to not report this. It's insane!"

"Yes, John, let's send in the Yard to deal with a wizard terrorist who they have no way of defending themselves against," Sherlock says. "As for the Ministry, what do you suggest we tell them? 'We don't know who Moriarty is, but he did steal some research on chemical weapons development and he's been spray-painting Albert Einstein quotes around the city, so we're fairly certain he's going to detonate a magical weapon of mass destruction.'"

John realizes that as per usual, Sherlock is infuriatingly right. The LEP wouldn't even take his rock solid tip about Masingberd seriously without Mycroft's intervention. They'd laugh him out of the office with this. Not to mention the fact that they'd be as likely to arrest Sherlock in connection with the clues they currently have as do anything else. John swallows. "I'll get my gun."

* * *

For reasons John doesn't even try to fathom, Sherlock says he will check out the homes of the first two chemical plant murder victims, and sends John to check the flat of Gerald Halbrud, the fourth victim, and the factory where John and Sherlock confronted the killer. Halbrud's flat is a washout. The block of flats is entirely quiet- it is rather late in the evening- and the man who answers the door for John and his imaginary charity organization is irritable but otherwise fine. John moves on to the factory.

It looks perfectly quiet as he prowls around the outside. No one appears to be on the property, but John checks each of the doors, just to be sure. He's jiggling the handle of the side door he entered last time he was here when a voice behind him calls, "Oi, you, what are you doing?"

Shit. John turns his back to the door and raises his empty hands to the rapidly approaching security guard, who's already reaching to his side for a weapon. "Sorry, mate, I was supposed to meet someone near here, I thought they'd nipped inside. No harm done, right?"

"No, I suppose not," the guard says, and whips his hand out of his jacket, a wand clenched in his fist. John tries to dodge sideways as he grabs for his gun, but the guard is far too close to miss.

John has never been stupefied before. It's not like falling asleep, or like being hit in the head. It's more like someone flicking a switch to turn his brain off, then back on, and John is opening his eyes somewhere completely different. He barely has time to think _indoors, carpet, supine, where?_ before someone hits him with an entirely different curse. " _Imperio_!"

A heavy lassitude floods John's limbs, muscles he didn't even realize were tense relaxing and releasing. His emotions are still there, the anxiety and the building anger, but it's like they've been turned all the way down and he's infused with a slight sense of euphoria. It reminds him of his recovery from the gunshot, when they were weaning him off the morphine and had him on the lowest possible dose: everything has become light and vaguely dreamy. John can't feel any injuries, but he has evidently been searched; his gun is no longer a comforting pressure in the small of his back.

"Get up," the someone says, and John finds himself rolling to his side and then standing up without feeling the slightest bit of intent to do so. It's a very odd sensation, like there is someone seated in his forebrain piloting his body, and John is just a passenger watching through the eye holes. As John gets up, he is able to see that he is in a small, plain office occupied by a battered desk, two chairs, and a filing cabinet. There is a stocky man with a shaved head leaning against the wall by the (closed) door with his arms folded over his chest. It's the security guard from outside, now wearing black slacks, wingtips, and a short-sleeved shirt, along with black gloves and a tactical vest.

"Come sit down." John's body turns him to face the man who's speaking: he notices the suit and tie first, because the man is impeccably dressed in what John is willing to bet are designer clothes. His eyebrows are thin, precise arches jagging across a face that ought to be expressive, ought to be _attractive_ based on his features, but somehow seems very flat. His mouth is a line- not grim, just absent of emotion. And his eyes...there's something wrong with his eyes. It's like the thousand-yard stare in reverse, a gaze at once intensely focused and yet so introspective that John gets the sense the man is thinking about what he looks like underneath his skin.

He's leaning against the desk and gesturing to a folding chair in front of him with his wand, and John's body walks over and sits itself in the chair. The man turns his back to John and walks casually around the desk to sit in the chair behind it, tucking his wand inside his jacket. He's probably trying to prove a point, because despite a desperate internal urge to leap out of his chair and grab the man, despite his fully-formed plan to seize the man by the scruff and swing him around as a shield so he can go after the man by the door, John is unable to do more than twitch his fingers in response to the thought.

The man with the wrong eyes regards John quietly for a moment, his hands folded on the desk before them. "John Watson," he finally says, speculatively. "Doctor John Watson." Is there a sneer there? "John. John-boy. Johnny." The man smiles very slightly, and in combination with the eyes it's absolutely chilling.

"You look so vacant, Johnny," the man says. "I might give you the benefit of the doubt and think it was the spell, if I hadn't seen you look that way so many times before." He studies John for another moment. "Why does Sherlock put up with you? You're no better than a Muggle; you kill for him, but so could anyone. I suppose you were easier to house train than a puppy, so there is that." John ignores the insults, stuck on the second sentence and what it implies- has this guy been watching him, watching _them_?

"Do you know who I am? Have a guess." Syrupy and sweet, the voice sing-songs mockingly at him. It's the same tone of voice Harry used when he was nine and she was taunting, _Do some magic, let's see it then_. "You can speak freely, by the way," the man adds as an afterthought.

"Moriarty," John says. Of course it's always possible Sherlock has more than one lunatic stalking him, he may in fact have a whole fan club full of them, but John's going with the odds on this one.

It seems he's right, since the man gives John a little golf clap and says, "Good _boy_. And look, I even have a treat ready for you." The man by the door steps up behind John's shoulder. He can hear him coming and it drives all his combat instincts wild to sit still while an armed and hostile man approaches him from behind, but John is completely incapable of turning around. A black-gloved hand comes down over his shoulder and offers him a Browning Hi-Power. _His_ Browning. John's breath hitches.

"A man needs a gun he can rely on," Moriarty says. "Take it."

"I don't want it," John says, because Moriarty said that he could talk, but his body is still bound to obey and he takes the gun and holds it loosely in his left hand. "What is this about? All this?" John asks.

Moriarty just looks at him. "You know what it's about," he says. And John does: it's about Sherlock, of course. The messages, maybe even the murders, were all apparently directed at him. It was sort of a stupid question; John's not sure what he expects Moriarty to give away. The man is a raving nutter. If John's damn arms would just obey him, he could put a bullet in his head and not feel a shred of guilt about it.

"He's going to stop you. Regardless of what you do to me," John says. "You must know that."

The small, tight smile again. "Oh Johnny, don't be tedious. You make the mistake of thinking Sherlock Holmes is some paragon of virtue. You have no idea how wrong you are."

"Fuck you," John snaps. "I'd like to show you to the Yard, or the Aurors, so they get an idea of what real psychopathy looks like, you sick-"

"Stop talking," Moriarty says, and John's voice abruptly cuts out. "Seb?" The man behind John passes down another item: John's cell phone. "You're sending Sherlock a text," Moriarty says. "These words exactly. Chemeuro plant, comma, bomb on factory floor, full stop. Moriarty is here, comma, notify Aurors, full stop. Hyphen, J, W, full stop." John sets the gun in his lap so that he can punch out the message on the keyboard. "Send it." John does, and it almost immediately begins to light up with a flurry of incoming texts from Sherlock. John watches the clock on the phone count two minutes, then three. "Turn the phone off and put it away," Moriarty says.

Surely Moriarty has overplayed his hand; but then John remembers Sherlock saying _This place was an air raid shelter,_ and realizes that Sherlock will assume that John has gone onto the factory floor and that the cell signal dropped, not that John turned his phone off. He knows Sherlock well enough to see what will happen next. He's not going to call the Aurors, because when has he ever, he's going to rush straight here to confront Moriarty himself, the _fucking idiot._

Moriarty stands up, rubbing his hands together briskly. "Well," he says brightly. "That's done. Shall we set up the board?"

They walk him down to the factory floor. "Seb" hands him an earpiece, and Jim gives him instructions on where to stand, how to act, what to do when Sherlock comes in. Moriarty walks off across the factory floor, no doubt to find a quiet corner to lurk in, and Seb heads for the catwalks. John occupies the few minutes of waiting with elaborate fantasies about how he's going to murder them both as soon as he gets out from under this spell. It's better than the other option, which is panicking over what he's about to do. There are so very many ways it can end badly for John, for Sherlock. He wonders what Moriarty will do if Sherlock reacts too fast and kills John before he realizes what's going on. Laugh, probably.

John's muscles are still on autopilot, so it doesn't really matter that he's not ready when the double door clangs open and Sherlock walks in. Something in John's gut twists sideways with guilt, because this is really, really going to hurt.

John strides out in front of Sherlock, raising his gun arm at the same time, and points the Browning at Sherlock's center of mass. "Stop," John barks. "Show your hands." Sherlock obeys instantly, his eyes widening. John is out of preset commands now, and he has to wait for Moriarty to feed him his lines through the earpiece before he can speak.

"You're surprisingly gullible," John says. "I don't know how you've survived this long." Sherlock's face shows horror as he jumps to the wrong conclusion, the one Moriarty intended. It makes John furious that Sherlock is that quick to believe John capable of this. Or is it a backhand sort of compliment, that he's ready to believe John is an evil genius?

Next line. "This is where I first showed my hand," John says. "Were you as surprised that day as you are now?"

"John, stop. What is this?" Sherlock's voice is rough.

"I hope you're not bored already," John says. "Don't worry. It's about to get a lot more interesting."

John can tell the exact moment when Moriarty steps into view, because Sherlock's eyes shift instantly. Sherlock goes for his wand as the footsteps approach behind John, and John responds to the mutter in his earpiece and harshly says, "Don't."

"I'd do what he says," Moriarty drawls, stopping just inside the line of John's peripheral vision. "He's the man with the gun, after all."

"What is this?" Sherlock says again, his eyes darting rapidly between the two of them.

"This is a game," Moriarty says. He puts his hands in his pockets and smiles, rocking on the balls of his feet. "I call it 'Jim says.'" The smile vanishes and the flat, mental patient expression is back with a vengeance. "Jim says- Johnny, cock the gun." John thumbs the safety off and pulls the slide back to chamber the first round. "Jim says- Johnny, put the gun in your mouth." Dawning comprehension and deeper horror on Sherlock's face as John inserts the barrel between his lips; he's done it before, a ritual and a gesture of control, but he's not in control now and this is not the same thing as contemplating suicide in his MOD flat, not at all. "Johnny, pull the-"

"Stop!" Sherlock rasps, desperate, and Moriarty does. John wants to gasp with relief, but he literally, physically can't.

"Oops!" Moriarty says, his voice high and teasing. "I didn't say 'Jim says'!"

"Moriarty," Sherlock almost snarls the name, but then his face suddenly changes. The momentary rage disappears, and Sherlock goes cold and impassive. "This is beneath you. If you want to fight, get out your wand and we'll do this properly." He keeps darting his eyes at John, who still has the gun pressed into the roof of his own mouth.

"Oh, _Sherlock_ ," Moriarty says, his voice rich with disappointment. "A duel? It would last about as long as it takes to say six syllables. Boring."

"Then why don't I just get out mine," Sherlock suggests.

Moriarty clicks his tongue. "Johnny, tell him why not. And put the gun back on Sherlock, if you please."

With a surge of relief, John pulls the gun from his mouth. "Because there's another wizard in the catwalks." He levels the weapon at Sherlock, who ignores it and focuses his attention on Moriarty.

"You wanted me, and here I am," Sherlock says. "Are you planning to tell me why, or am I supposed to guess?"

"I just fancied a little chat," Moriarty says. "It was about time, don't you think?" He seems to notice Sherlock scanning as much of the factory floor as he can see without looking away from Moriarty. "Oh, there's a bomb," Moriarty says. He pats his jacket pocket. "But that's rather besides the point."

"So what is the point," Sherlock says stiffly.

"Simply this," Moriarty says. "Come in, or fuck off."

"I don't think I fancy either of those options," Sherlock says levelly, looking back at Moriarty now.

"Those are the options you get. Don't mistake me, I would find destroying an intellect of your caliber _extremely_ disappointing. You know yourself, how rare it is to meet someone you can truly consider a peer." Moriarty's grin is sharkish. "And, of course, I am a great admirer of your...early work." John has no idea what this is supposed to mean, but it causes Sherlock to flinch noticeably.

"So we're both men of extraordinary ability. And your conclusion is that we should join forces? Tear down the barrier between the Muggle and wizarding worlds?" Sherlock's voice drips with scorn.

"You can't deny that you think about it," Moriarty says. "Not power, or money, not for Sherlock Holmes. But the excitement. The _challenge_. True intellectual freedom. Truly worthy companionship."

"No," Sherlock says simply.

A scowl passed briefly over Moriarty's features, and is gone. "You can have a few moments to think about it," he says. "I'm not as demanding as certain other parties."

"I said I'm not interested," Sherlock says. "Surely you aren't foolish enough to think emulating his methods will endear you to me."

"I wonder," Moriarty says softly. "Have you told your pet about it? About him?" He pulls a small black box from inside his jacket and turns it once in his hands. It is simple and non-theatrical, just a square of plastic with a single button in its center. John and Sherlock's eyes immediately focus on it. 

"No," Sherlock says again, unwavering, and despite the insane danger John loves him for it.

"Well. If that's your final answer, then." Moriarty smiles. And he presses the button.

There's a muffled bang deep inside the piece of machinery they are standing beside, and a murky cloud of green mist begins to spill out of the apertures in the machine. Moriarty and Sherlock are standing closest, and the cloud curls around them first, the edges plucking at their clothes and hair like wispy fingers.

"What did you use, Jim," Sherlock says. It's not a question.

"Sherlock," Moriarty sighs. "Why do you keep asking questions you know the answers to?" The cloud is closing on John; it doesn't seem to have any effect on Sherlock or Moriarty, but John is not really into taking chances with chemical weapons, and he takes a deep breath and holds it.

Sherlock smiles, suddenly: the real smile, and it's so incongruous that John almost laughs. "You got the formula wrong," Sherlock says.

"Perhaps," Moriarty admits. "A shame. I suppose it's fortunate that we have an additional test subject." Then _he_ smiles. "Inhale, Johnny."

John thinks _oh shit_ and takes a deep breath of green vileness. He's suddenly in the middle of the deepest, blackest depression he's ever felt; he's momentarily distracted from it by a sparking of white flares in his peripheral vision, and then he becomes aware of the gun in his hand and thinks _why bother_. He drops it clattering to the ground. Through the murky green fog, he sees Sherlock grab out his wand and fling a furious " _Stupefy_!" in Moriarty's direction. Then Sherlock's right there, grabbing John by the jacket and physically dragging him away from Moriarty, away from the gas cloud, which is already dissipating.

" _Expelliarmus_!" barks Seb from the catwalks- he must have incredible range to be able to cast that far - and the wand flies from Sherlock's hand. John staggers a few more steps and stumbles, falling to his knees. Out of the gas, his head is starting to clear and he's capable of thinking of his gun, but it's all the way over by Moriarty. Who now has his wand in his hand. Perfect.

Sherlock stands over John, fists clenched. Moriarty tuts reprovingly. "Well, this has been an evening of learning experiences," he says. He starts to turn away, towards, the door; Sherlock's posture tightens even further, but he glances up at the catwalks and then doesn't move. "Ah, that reminds me." Moriarty turns back to Sherlock and smiles widely. "Be a love and roll up your sleeve." 

Sherlock's mouth drops slightly open and his face twists, turning chalk white. It's like the expression he wore when he first saw John holding a gun on him, but about a hundred times worse. For the first time this evening, John is actually afraid. He has no idea what about Moriarty's words has caused this reaction in Sherlock, and he does not want to know. He doesn't.

Sherlock's voice is hoarse and choking, as if he's being strangled by the words. "I refuse. And I'll remind you I'm an Occlumens; you won't find me easy to control."

"Well that's not true, is it?" Moriarty says teasingly. He raises his wand hand and calls lightly, " _Crucio_!" The strength of the malice behind it hits John like a sledgehammer, smashing him to the floor, and he writhes, screaming. He thought nothing would ever hurt as much as being shot had hurt; he was very, very wrong. This is like taking a bullet in every major nerve cluster in his body, all at once. It's like his entire body is bathed in flame, and he's rolling on the floor as helplessly as if it is flame and he can smother it, except that he can't. Nothing on earth can extinguish this fire and for one long, bright moment the only thought in his head is not _Please god let me live_ but _Please god let me die._

"All right!" Sherlock is shouting to be heard over John. "Stop it! All _right!_ "

The pain stops and John sobs once with relief. His face is wet with tears. He turns his head to see that Sherlock has turned toward Moriarty and is fumbling with his right sleeve. "No, Sherlock," Moriarty says gently. "The left one." Sherlock closes his eyes. "Sit up, Johnny, you want a good view, don't you?"

John obediently rolls himself upright, angrily wiping his face on his sleeve, shaking with aftershocks; it takes him a second to realize that he obeyed out of shock and habit, that his body is now his own again, but he feels far too shattered to do anything dramatic. Sherlock unfastens the buttons on his left shirt cuff and rolls up the sleeve, his lips pressed in a grim line. 

John can barely think, his head is still so full of the echo of pain, but something about this is ringing against his memory. Sherlock furious and horrified and resisting. The left arm, why is the left arm significant? Has John ever seen Sherlock's bare left arm? Always long-sleeved dress shirts, he sleeps in a t-shirt but always with a dressing gown over it, nicotine patches are always, always on the right arm. Why doesn't Sherlock want John to see his left forearm?

The Aurors' distrust of Sherlock, and Dye's warning. The glint of admiration in Sherlock's eyes when he saw the bullets filled with basilisk venom. Mycroft's words, _Not until I had ascertained the extent of Sherlock's involvement_. Oh God, oh Christ.

"Show him," Moriarty says. Sherlock doesn't move. " _Show him_!" Sherlock pivots to face John, but he won't look him in the eye. The expression on his face is awful as he raises his left arm and turns his wrist, so that John can see what he's just begun to realize must be there.

The scarring is upraised, and so white that it stands out even against Sherlock's fish-pale forearm. It looks exactly like the photographs John's seen, only in white instead of black, upside-down since Sherlock is holding his arm upright: the leering skull, the protruding snake looped into a double knot that seems to mirror and mock the mathematical symbol for infinity, the snake's head rearing back with fangs displayed, about to strike. The Dark Mark. It's the fucking _Dark Mark_. John's brain gibbers.

"I'll just leave you to it then!" Moriarty chirps. John doesn't see him leave, to be honest. He's staring at Sherlock's arm, and he should be thinking something, but his mind is just blank. The utter silence of the factory floor blurs in his ears. His numbly hauls himself upright, fighting his right leg as it shivers and falters under him. When he bends to pick up his gun, he stumbles as it almost collapses. Sherlock, who has been standing stock still and silent, takes two hesitant steps toward him before John regains his balance.

John’s glance skims the catwalks but there’s no sign that anyone remained behind. It’s just John, Sherlock, and the white noise filling up his head. The atmosphere on the factory floor still has a sort of residual dread, and suddenly John can’t stand being inside another moment. He makes for the double doors, for the free air outside and _home._

John doesn’t feel the cold, even though the great deep breaths he’s taking leave frosty clouds hanging on every exhale. Behind him, he hears Sherlock catch the door before it closes, step briskly through and then stop. John doesn’t want to look, doesn’t need to see it again, but he does need to see the expression on Sherlock’s face: is it horror, revulsion, guilt? Anger?

Sherlock looks strange with no jacket and his left sleeve open; he’s usually wholly put together and now he looks half taken apart. But when John turns to look at him, after he manages to rip his eyes away from the Mark again and look at Sherlock’s face, there’s no expression there at all. Completely unreadable.

“Sherlock-” John starts, and then stops. He waits for Sherlock to tell him any one of a thousand things that would make this all right: it’s an illusion, a spell, a trick by Moriarty, some plan of Sherlock’s, or John is simply mistaken. Sherlock never hesitates to tell John he’s mistaken, never hesitates to explain.

But here and now, in answer to the question John hasn’t quite dared ask aloud, Sherlock Holmes doesn’t say anything at all.

**Notes for the Chapter:**

> Dee-light has illustrated [this final scene](http://archiveofourown.org/works/463624)\- thank you!


End file.
